U.S. Department of Justice

Office of Justice Programs

National Institute of Justice

National Institute of Justice

Risk Factors and Indicators Associated 

With Radicalization to Terrorism in the 

United States: What Research Sponsored by 

the National Institute of Justice Tells Us

Allison G. Smith, Ph.D.

June 2018

This paper was prepared with support from the National Institute of Justice, Office of Justice Programs,

U.S. Department of Justice, under contract number 2010F_10097. The opinions, findings, and conclusions or

recommendations expressed in this publication are those of the authors and do not necessarily represent

those of the Department of Justice.

NCJ 251789
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The National Institute of Justice is the research, development, and evaluation agency of the U.S. Department of Justice. NIJ’s 

mission is to advance scientific research, development, and evaluation to enhance the administration of justice and public safety. 

The National Institute of Justice is a component of the Office of Justice Programs, which also includes the Bureau of Justice 

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Opinions or conclusions expressed in this paper are those of the authors and do not necessarily reflect the official position or 

policies of the U.S. Department of Justice. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States i 

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Executive Summary 

Since its creation in 2012, the National Institute of Justice’s Domestic Radicalization 

to Terrorism program has sponsored research to support community members and 

practitioners in (1) identifying individuals who are radicalizing to terrorism and (2) 

developing prevention and intervention efforts. Although several of these projects are 

ongoing, important findings regarding the potential risk factors and indicators1

 associated 

with engaging — or attempting to engage — in terrorism have begun to emerge. As many of 

these findings were based on analyses that compared individuals who engaged or attempted 

to engage in terrorism with those who did not, there is increasingly strong evidence that 

these risk factors and indicators may help practitioners to distinguish between individuals 

who are more or less likely to attempt to conduct a terrorist attack, as well as provide a 

foundation for prevention and intervention programs. 

Potential risk factors that were identified in research that included both group-based and loneactor terrorists of varied ideologies (as well as nonviolent comparison groups) are listed below. 

While some of these risk factors are static, meaning that they cannot change over time 

(e.g., having a criminal history), others can change and, thus, suggest possible avenues 

for prevention and intervention efforts. Specifically, depending on which risk factors are 

relevant in individuals’ particular cases, efforts that focus on addressing terrorist beliefs, 

treating mental health issues, improving individuals’ employment or educational prospects, 

and/or helping them to develop and sustain positive relationships with nonextremist and 

nondelinquent peers may contribute to prevention and intervention strategies. 

Potential risk factors that were identified in research that included only lone-actor terrorists 

are listed on page iii below. As can be seen, there is some overlap between these risk factors 

and those found in the analyses of both group-based and lone-actor terrorists. Specifically, 

having a criminal history, having mental health issues (or having received a diagnosis of 

schizophrenia or delusional disorder), being unemployed, being single, being a loner (or 

1

 A risk factor generally increases the likelihood that a certain outcome will occur, whereas an indicator generally 

helps to assess whether it is occurring. 
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socially isolated), and having military 

experience2 were identified as potential 

risk factors for engaging or attempting to 

engage in terrorism in both sets of analyses. 

In addition, having personal and political 

grievances, having an enabler who assists 

in planning an attack or provides the 

inspiration for it, having at least a bachelor’s 

degree, living alone, and being male were 

also identified as potential risk factors 

among lone actors.3 

Taken together, these findings again 

suggest that efforts that focus on treating 

mental health issues, improving individuals’ 

employment prospects, and/or helping 

them to develop and sustain positive 

relationships may contribute to prevention 

and intervention efforts. They also provide 

some evidence that these efforts may need 

to take into account individuals’ personal 

and political grievances. 

Although fewer potential indicators 

associated with engaging or attempting 

to engage in terrorism were identified in 

NIJ-sponsored research — and only one 

of these was based on an analysis that 

included a nonviolent comparison group — 

the findings tentatively suggest that those 

who are closest to individuals may be well 

positioned to observe some of the potential 

warning signs that they may be moving 

toward terrorism. Specifically, lone actors 

who engaged or attempted to engage in 

terrorism tended to broadcast their intent 

to do so, as well as convey information 

about their grievances, extremist ideologies, 

Potential Risk Factors Associated With Engaging or Attempting to Engage in Terrorism 

Among Both Group-Based and Lone-Actor Terrorists in the U.S. 

Having a History of Criminal Violence 

Having a Criminal History 

Having Been Involved With a Gang or Delinquent Peers 

Having a Terrorist Friend 

Being a Member of an Extremist Group for an Extended Period 

Having a Deep Commitment to an Extremist Ideology 

Having Psychological Issues 

Being Unemployed 

Having a Sporadic Work History 

Having Less Education 

Having a Lower Social Economic Status 

Failing to Achieve One’s Aspirations 

Having Trouble in Romantic Relationships 

Having Trouble in Platonic Relationships 

Having Been Abused as an Adult 

Being Distant From One’s Family 

2

 The finding that having military experience was associated with engaging or attempting to engage in terrorism 

is possibly related to individuals’ capabilities or perceived capabilities of carrying out an attack. 

3

 Since the potential risk factors related to (1) having personal and political grievances and (2) having an enabler 

were not identified based on analyses that included nonviolent comparison groups, they should be considered 

tentative. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States iii 

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Potential Risk Factors Associated With Engaging or Attempting to Engage in Terrorism 

Among Lone-Actor Terrorists in the U.S. 

Having a Criminal Record 

Having Personal & Political Grievances 

Having Received a Diagnosis of Schizophrenia or Delusional Disorder 

Having an Enabler 

Being Unemployed 

Having at Least a Bachelor’s Degree 

Being Socially Isolated 

Being Single 

Living Alone 

Having Military Experience 

Being Male 

or desire to hurt others. It is, however, 

important to note that these findings were 

not based on analyses that included a 

comparison group, and thus it may be the 

case that there are even more individuals 

who convey similar information but who do 

not go on to engage or attempt to engage in 

terrorism. Thus, these types of statements 

cannot be assumed to be either necessary 

or sufficient evidence that the individuals 

who make them will follow their words with 

action. Finally, two potential indicators 

possibly related to planning or preparing 

for attacks — attending more extremist 

group meetings and stockpiling weapons — 

were also identified. 

When comparing the potential risk factors 

and indicators identified in NIJ-sponsored 

research with the risk factors and/or 

indicators included in three existing 

instruments and methods developed to 

assess the risk of individuals engaging 

in extremist crime (The Extremist Risk 

Guidelines; ERG 22+), extremist violence 

(The Violent Extremist Risk Assessment, 

Consultative Version 2; VERA 2), or general 

violence (The Historical Clinical Risk 

Management-20, Version 3; HCR-20v3), 

there was substantial overlap. 

As can be seen on the next page, nine 

out of 10 of the categories of potential 

risk factors and indicators identified in 

NIJ-sponsored research are included in at 

least one of the existing risk assessment 

instruments or methods examined. 

There are, however, some distinctions in 

which categories are included based on 

whether these instruments and methods 

focus on assessing the risk of general 

violence versus extremist crime and/or 

terrorism. Specifically, risk factors and 

indicators related to having (1) trouble in 

professional, educational, and economic 

domains and (2) trouble in relationship 

domains are included only in the method 

developed to assess the risk of general 

violence (the HCR-20v3); and risk factors 

and indicators related to having (1) 

extremist associates, (2) an extremist 

ideology and/or grievances, and (3) skills 

and resources for committing violence 

are included only in the instruments and 

methods developed to assess the risk of 

extremist crime or violence (the ERG 

22+ and VERA 2). Overall, these findings 

suggest that while many of the risk factors 

and indicators associated with engaging in 

terrorism may not be that different from 

those associated with engaging in violence 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States iv 

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x 

x 

Comparison Between Categories of Potential Risk Factors and Indicators Identified in 

NIJ-Sponsored Research and Those Included in Existing Risk Assessment Instruments 

and Methods 

Category ERG 22+ VERA 2 HCR-20V3 

Past History of Violence x 

Past History of Criminal Activity/Delinquency x 

Extremist Associates x x 

Extremist Ideology/Grievances x x 

Intent to Commit Violence x 

Skills/Resources for Committing Violence x x 

Mental Health Issues x 

Trouble in Professional/Educational/Economic Domains 

Specific Demographic Characteristics 

more generally — and, thus, programs 

developed to mitigate the risk of violence 

among individuals not motivated by 

extremist ideologies may have relevance to 

those who are — risk factors and indicators 

related to having extremist associates, 

extremist beliefs, and skills and resources 

for committing violence may also need 

to be considered when developing risk 

assessment protocols as well as prevention 

and intervention programs. 

Finally, there are several limitations to 

this research that need to be kept in 

mind, including those related to missing 

or potentially inaccurate data, the need 

to make judgments that are sometimes 

subjective, and the likelihood that some 

of the individuals included in the various 

analyses overlap. It is also essential to note 

that not all individuals who experience or 

are exposed to risk factors — or exhibit 

indicators — associated with engaging 

or attempting to engage in terrorism go 

on to do so. While research that analyzes 

samples of terrorists can help to identify 

characteristics, experiences, and behaviors 

that are generally associated with an 

outcome, further work is required to 

determine whether these characteristics, 

experiences, and behaviors are relevant in 

the case of a particular individual. 

x 

x 

x
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Risk Factors and Indicators 

Associated With Radicalization to 

Terrorism in the United States: What 

Research Sponsored by the National 

Institute of Justice Tells Us 

Introduction 

Since its creation in 2012, the National Institute of Justice’s Domestic Radicalization 

to Terrorism program has sponsored research to support community members and 

practitioners in (1) identifying individuals who are radicalizing to terrorism4 and (2) 

developing prevention and intervention efforts.5 To do so, it has funded top social and 

behavioral science researchers from around the world to examine risk factors, protective 

factors, and indicators associated with radicalization to terrorism in the United States. 

These research teams have studied a wide range of groups and individuals who support 

and commit ideologically motivated terrorism to further political, social, or religious goals, 

including Islamist terrorists but also those associated with anti-government, anti-capitalist, 

nativist, and other political and social terrorist movements (often referred to as “religious,” 

“left-wing,” “right-wing,” and “single-issue” terrorism in the literature). 

Although many of these projects are ongoing, important findings regarding the 

potential risk factors and indicators associated with individuals engaging — or 

4

 There are different definitions of violent extremism, radicalization, and terrorism, but, for the purpose of this 

paper, terrorists are those individuals who support or commit ideologically motivated violence to further political, 

social, or religious goals; radicalization is the process by which individuals enter into terrorism; and terrorism is 

an act that involves the threatened or actual use of ideologically motivated violence to further political, social, or 

religious goals. 

5

 For the purpose of this paper, prevention efforts focus on addressing the broader underlying conditions that 

may be associated with radicalization to terrorism. (These would be referred to as “primary prevention” efforts in 

the public health literature.) Intervention efforts focus on either (1) identifying individuals who are potentially at 

risk for radicalizing to terrorism and developing tailored programs to mitigate this risk (“secondary prevention” 

efforts) or (2) rehabilitating those who have already radicalized (“tertiary prevention” efforts). 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 2 

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attempting to engage — in terrorism have 

begun to emerge.6,7 This paper begins 

by providing a brief background on risk 

factors and indicators, how they were 

identified in NIJ-sponsored research, and 

some general limitations that need to be 

considered when interpreting the results 

of these analyses. It then presents findings 

based on four NIJ-sponsored projects and 

some of the possible implications of these 

findings for prevention and intervention 

efforts. Next, it compares these findings 

with the risk factors and indicators 

included in three existing risk assessment 

instruments and methods that have been 

developed to assess the likelihood that 

individuals will engage in illegal extremist 

activity, terrorism, or violence in general. 

Finally, it concludes by providing an 

overview of the findings and discussing 

next steps. 

Background 

Risk Factors 

In the most basic sense, a risk factor is 

something that increases the likelihood that 

a certain outcome will occur, in this case 

that an individual will engage or attempt 

to engage in terrorism. In other words, 

individuals who have experienced or 

are exposed to the potential risk factors 

identified in the NIJ-sponsored research 

discussed below may be more likely to 

engage or attempt to engage in terrorism. 

It is important to note, however, that this 

does not mean that everyone who has 

experienced or is exposed to a relevant 

risk factor — or even several relevant risk 

factors — will engage or attempt to engage 

in terrorism. Radicalization to terrorism is 

a complex process that has been shown to 

vary by the extremist ideologies individuals 

embrace, the time periods in which they 

radicalize, the groups or movements they 

join, and individuals’ own characteristics 

and experiences (Hamm & Spaaij, 2015a; 

Horgan et al., 2016; Klausen et al., 2016; 

LaFree, 2015).8

 While research that 

analyzes samples of terrorists can help to 

identify characteristics and experiences 

that generally increase the likelihood 

that individuals will engage or attempt to 

engage in terrorism, further investigation 

is required to determine whether these 

characteristics and experiences are relevant 

in the case of a particular individual. 

Further, engaging or attempting to engage 

in terrorism is a very rare activity, and as 

will be seen below, several of the risk factors 

that have been identified by the research 

teams are shared among substantial 

portions of the general U.S. population. 

While, arguably, specific combinations of 

risk factors are shared by fewer individuals, 

predicting with any accuracy who will 

engage or attempt to engage in this very 

rare activity is an unrealistic goal. On the 

other hand, using risk factors to assess the 

relative likelihood that individuals who have 

already raised concerns may engage or 

attempt to engage in terrorism may be more 

6

 This paper focuses on risk factors and indicators associated with engaging or attempting to engage in terrorism 

as opposed to those associated with (1) engaging in nonviolent extremist crime or (2) endorsing extremist beliefs 

but not engaging in any type of illegal activity. Further, it focuses on risk factors and indicators associated with 

individuals (versus groups) engaging or attempting to engage in terrorism. 

7

 Although NIJ is also sponsoring research to identify protective factors that may decrease the likelihood of 

individuals radicalizing to terrorism, this research is in an earlier stage and will be discussed in a future paper. 

8

 NIJ-sponsored research is also exploring risk factors, protective factors, and indicators associated with engaging 

in extremist crime and terrorism among individuals who embrace specific extremist ideologies (e.g., Islamist 

terrorism), but this paper focuses on risk factors and indicators that are common across individuals who embrace 

a range of extremist ideologies. 
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feasible. Additionally, the identification of 

valid risk factors can help to inform the 

development of prevention and intervention 

programs focused on mitigating the threat 

of terrorism.9 

Indicators 

Whereas risk factors may increase the 

likelihood that a certain outcome will occur, 

indicators may help to assess whether it is 

occurring. To give an example, although 

thinking poorly of others and viewing 

violence in a positive manner may be risk 

factors for engaging in bullying, frequently 

getting into physical or verbal fights may 

be an indicator of it. Indicators may also 

be used to assess whether an individual is 

experiencing or being exposed to a specific 

risk factor. Using the same example, 

making frequent comments that seem to 

promote violence may be an indicator of an 

individual’s positive attitude toward it. 

Similar to risk factors, it is not expected 

that every individual who exhibits one 

or more potential indicators associated 

with an outcome is necessarily engaging 

in or experiencing it (e.g., many children 

who get into physical or verbal fights are 

not bullying others). On the other hand, 

when used judiciously, the identification 

of evidence-based indicators associated 

with engaging in an activity such as 

terrorism may help community members 

and practitioners to identify and stop 

these activities before they reach their 

final stages. 

Identifying Potential Risk Factors 

and Indicators Associated With 

Engaging or Attempting to Engage 

in Terrorism 

Numerous risk factors and indicators 

potentially associated with whether an 

individual engages or attempts to engage 

in terrorism have been identified in NIJsponsored research projects (Hamm & 

Spaaij, 2015a; Hamm & Spaaij, 2015b; 

Horgan et al., 2016; P. James, personal 

communication, February 4, 2016; Jasko, 

LaFree, & Kruglanski, 2016; Sawyer, 

2016; B. Smith, personal communication, 

April 8, 2016; Smith et al., 2016). In some 

projects, these risk factors and indicators 

have emerged from analyses that compare 

extremists who engaged or attempted to 

engage in terrorism with extremists who 

did not. In these cases there is evidence that 

the hypothesized risk factors and indicators 

are more likely to be associated with 

individuals who engage in this behavior 

and, equally important, are less likely to be 

associated with individuals who do not. As a 

consequence, there is stronger evidence that 

they may help practitioners to distinguish 

between extremists who are more and less 

likely to attempt or conduct a terrorist 

attack as well as provide a foundation for 

the development of intervention programs.10 

9

 Again, the importance of identifying protective factors that can inform risk assessments and the development of 

prevention and intervention programs should not be underestimated. 

10 It should be noted that the composition of an ideal comparison group depends on the purpose of the analysis 

being conducted. For example, when the goal of an analysis is to inform practitioners responsible for assessing 

whether an individual may pose a threat of engaging in terrorism in order to advise criminal justice agencies on 

appropriate follow-up actions (e.g., pursuing further investigation, engaging in specific tailored interventions, 

rehabilitating or releasing individuals who have been incarcerated), arguably it would be most appropriate to 

compare individuals who have engaged or attempted to engage in terrorism with other extremists who have 

come to law enforcement or public attention but who have not engaged or attempted to engage in terrorism. 

If, on the other hand, the goal of an analysis is to inform community members and practitioners interested in 

understanding characteristics and experiences that may make an individual more vulnerable to radicalization to 

terrorism in order to develop general prevention and/or early intervention efforts, it may be more appropriate 

to compare individuals who have engaged or attempted to engage in terrorism with members of the larger 

population of nonextremists. 
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Other projects were not directly focused 

on distinguishing between individuals 

who do and do not engage or attempt to 

engage in terrorism, and thus they did not 

include nonviolent comparison groups. In 

these cases, this paper reports only those 

potential risk factors or indicators that 

meet at least one of two criteria: (1) there 

is concrete evidence that they are more 

prevalent among the terrorists studied than 

among the general U.S. population11 and/ 

or (2) they are associated with the majority 

(i.e., 50 percent or more) of individuals 

the researchers examined. If a risk factor 

or indicator meets the first criterion, there 

is evidence that it may, in combination 

with other risk factors and indicators, help 

community members and practitioners 

to identify those individuals among the 

general U.S. population who have a higher 

likelihood of — or are in the process 

of — radicalizing to terrorism. The risk 

factors identified may also suggest areas for 

prevention and early intervention programs 

to address. If a risk factor or indicator 

only meets the second criterion, its validity 

and corresponding utility are not as clear, 

although it still may be worthy of note.12 

Limitations of the Research 

When examining the findings discussed 

below, it is important to keep in mind 

that they are based on analyses that vary 

in terms of the terrorists being examined 

and with whom these individuals were 

compared (when there was a comparison 

group). While all of the research focused 

on extremists who were based in the U.S. 

and who embraced a range of extremist 

ideologies, the four projects differed in 

terms of the other criteria they used when 

developing their samples. The research 

conducted by the National Consortium 

for the Study of Terrorism and Responses 

to Terrorism (START) based at the 

University of Maryland examined the 

radicalization processes and trajectories of 

violent and nonviolent (both criminal and 

noncriminal) extremists and included both 

group-based and lone actors.13 Similarly, 

the research team led by the University 

of Arkansas collected data on violent and 

nonviolent members of extremist groups 

and lone actors, but this project only 

included individuals who had been indicted 

for terrorism or terrorism-related activities 

in U.S. federal courts.14 The two other 

11 Again, this type of general population comparison group would be more valuable when conducting analyses to 

inform the development of general prevention and/or early intervention efforts. 

12 To give an example, consider the finding that 41 percent of a random sample of individuals who engaged 

in unwanted approaches toward members of the U.S. Congress or their staffs between October 1998 and July 

1999 expressed a desire to harm or have harm occur to (i.e., threatened) the person they approached (Scalora 

et al., 2003). On the surface, this would seem to be an important indicator that an individual was preparing to 

engage in harmful activity. At the same time, however, the research team found that 65 percent of a random 

sample of individuals who did not engage in unwanted approaches toward members of the U.S. Congress or their 

staffs during the same time period (but who came to law enforcement attention) also threatened members of 

Congress or their staffs, a percentage that is substantially and significantly higher than the one associated with 

those who engaged in unwanted approaches. In other words, in this case those who did not express these types 

of threatening communications engaged in more unwanted approaches. That being said, given that a substantial 

proportion of individuals who engaged in unwanted approaches also expressed these types of communications — 

which, by their very nature, are concerning — it is highly unlikely that anyone would advise community members 

or practitioners to ignore them. 

13 The “Empirical Assessment of Domestic Radicalization Project” (NIJ Award #2012-ZA-BX-0005) is led by 

principal investigator (PI) Michael Jensen and conducted by a project team that includes Patrick James, Kasia 

Jasko ( Jagiellonian University, Poland), Gary LaFree, Daniela Pisoiu (University of Hamburg, Germany), Anita 

Atwell Seate, John Stevenson, and Herbert Tinsley. 

14 The projects “Identity and Framing Theory, Precursor Activity, and the Radicalization Process” (NIJ Award 

#2012-ZA-BX-0003) and “Sequencing Terrorists’ Precursor Behaviors: A Crime Specific Analysis” (NIJ Award 

#2013-ZA-BX-0001) are led by PI Brent Smith and conducted by a project team that includes Andy Brooks, Kelly 

Damphousse (University of Oklahoma), Kevin Fitzpatrick, Brent Klein, Paxton Roberts, David Snow (University of 

California, Irvine), and Anna Tan (University of California, Irvine). 
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Nonviolent 

Lone Actors/ Ideologies Comparison 

Project Team Group Actors Embraced Group Sample/Population 

START Both Varied Extremists 

who did not 

attempt to 

engage in 

terrorism 

This project randomly sampled 1,475 individuals from the 

larger population that met its inclusion criteria.18 It is the only 

project that also includes extremists who did not engage in 

any illegal extremist activity. The time period covered is from 

1965 to 2013. 

University of 

Arkansas 

Both Varied Extremists 

who did not 

attempt to 

engage in 

terrorism 

This project includes 1,373 individuals indicted for terrorism or 

terrorism-related activities in U.S. federal court between 1980 

and 2015. 

Indiana State 

University 

Lone Actors Varied N/A This project includes all 98 individuals who engaged in 

terrorism in the U.S. but acted alone, did not belong to a 

group, and were not directed by others. The time period 

covered is from 1940 to 2013. 

University of 

Massachusetts 

Lowell 

Lone Actors Varied N/A This project includes all 71 individuals who engaged in 

ideologically inspired violence in the U.S. in support of a 

broader group but who acted alone. The time period covered 

is from 1990 to 2013. 

projects — led by Indiana State University15 

and the University of Massachusetts 

Lowell16 — focused only on lone actors 

who had engaged in terrorism.17 Table 1 

includes some of the key characteristics 

of the individuals included in the projects 

highlighted in this paper. 

As can be inferred from Table 1, it is likely 

the case that some of the individuals 

included in the analyses conducted by these 

four NIJ-sponsored project teams overlap. 

For example, it may be the case that a lone 

actor who engaged in terrorism during the 

1990s would be included in all four projects 

and that, because of this, similarities 

between the projects’ findings may be 

due in part to the fact that they analyzed 

some of the same individuals. Although 

this possibility cannot be ruled out, when 

comparing the findings identified in the 

projects that included both group-based 

and lone actors with the projects that 

included only lone actors, it would seem less 

Table 1: Characteristics of U.S.-Based Extremists Included in Highlighted Projects 

15 The project “Lone Wolf Terrorism in America: Using Knowledge of Radicalization Pathways to Forge 

Prevention Strategies” (NIJ Award #2012-ZA-BX-0001) was led by PI Mark Hamm and conducted in collaboration 

with Ramon Spaaij (Victoria University, Australia). 

16 The project “Across the Universe? A Comparative Analysis of Violent Behavior and Radicalization Across Three 

Offender Types With Implications for Criminal Justice Training and Education” (NIJ Award #2013-ZA-BX-0002) 

was led by John Horgan (now at Georgia State University) and conducted by a project team that included Noemie 

Bouhana (University College London), Emily Corner (University College London), Paul Gill (University College 

London), and James Silver. 

17 The project conducted by the University of Massachusetts Lowell compared lone-actor terrorists with solo mass 

murderers. Because the latter also engage in criminal violence, they were not considered an ideal comparison 

group for the purpose of assessing the potential threat an individual posed. 

18 The project included individuals who radicalized in the U.S.; who espoused ideological motives and who 

acted on these motives; and who had (1) been arrested, (2) been indicted, (3) been killed in action, (4) been 

members of or associated with designated terrorist organizations, and/or (5) been members of or associated with 

organizations whose leaders or founders had been indicted for ideologically motivated terrorism. 
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likely that any similarities in their findings 

are due exclusively to an overlap in the 

individuals analyzed.19 

In addition to the potential overlap in the 

extremists included in the four projects, 

it is also important to keep in mind other 

limitations associated with collecting and 

analyzing the types of open-source data 

used in these projects. These include 

problems with missing data, which occur 

when researchers are unable to find any 

information (positive or negative) on 

the specific characteristics or behaviors 

they would like to analyze.20 In two of the 

projects discussed below, this resulted 

in individuals for whom data were not 

available being excluded from analyses, 

and in many cases, this led to findings 

that were based on only a small portion of 

the sample.21 As a consequence, it is not 

possible to assess whether the findings 

would be the same if data were available 

for all of the individuals included in these 

projects. Further, even when data were 

available for an individual, it may be the 

case that some of the information collected 

was not accurate. The project teams tried 

to minimize this possibility by drawing on 

multiple sources when collecting data, but 

given the clandestine nature of many of the 

activities they examined, some degree of 

inaccuracy seems inevitable. 

It is also important to consider that 

assessing whether an individual has 

experienced or been exposed to certain 

risk factors, such as having a deep 

commitment to an extremist ideology, may 

require subjective judgments on the part 

of researchers. The research teams tried to 

address this by having multiple individuals 

make these judgments independently and 

then comparing their assessments, but it is 

possible that different research teams might 

reach different conclusions. It could also 

be the case that a risk factor or indicator 

that one team found important to analyze 

was not viewed as important by other teams 

and, thus, was not analyzed. 

Findings From NIJ-Sponsored 

Research 

Potential Risk Factors 

Potential risk factors associated with 

engaging or attempting to engage in 

terrorism among both group-based and 

lone-actor extremists are displayed in Table 

2 (P. James, personal communication, 

February 4, 2016; Jasko, LaFree, & 

Kruglanski, 2016; Sawyer, 2016; Smith et al., 

2016; B. Smith, personal communication, 

April 8, 2016). 

All of the potential risk factors listed 

in this table are based on analyses that 

include a comparison group of extremists 

who did not engage or attempt to engage 

in terrorism. Specifically, these analyses 

involved calculating bivariate correlations 

or chi-square tests of independence to 

examine the relationship between each 

potential risk factor and engaging or 

attempting to engage in terrorism. In 

other words, each risk factor’s relationship 

19 Approximately 15 percent of the extremists included in the START project and approximately 6 percent of the 

extremists included in the University of Arkansas project were lone actors. 

20 Missing data may also occur when an individual who engaged in terrorism or nonviolent extremist activity is not 

included in a project because he or she has not been apprehended or otherwise identified. 

21 In the analyses that involved both group-based and lone actors (i.e., those conducted by START and the 

University of Arkansas), it was frequently the case that data were missing for more than 50 percent of the 

individuals included in the sample. In the analyses that involved only lone actors (i.e., those conducted by Indiana 

State University and the University of Massachusetts Lowell), if information about relevant characteristics or 

behaviors was not available for individuals, these individuals were included in the analyses, and it was assumed 

that they did not possess these characteristics or engage in these behaviors. 
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Table 2: Potential Risk Factors Associated With Engaging or Attempting to Engage 

in Terrorism Among Both Group-Based and Lone-Actor Terrorists in the U.S. 

Potential Risk Factor Identified By 

Having a History of Criminal Violence22 START 

Having a Criminal History23 START 

Having Been Involved With a Gang or Delinquent Peers START 

Having a Terrorist Friend START 

Being a Member of a Terrorist Group for an Extended Period of 

Time24 

University of Arkansas 

Having a Deep Commitment to an Extremist Ideology START 

Having Psychological Issues25 START 

Being Unemployed START 

Having a Sporadic Work History START 

Having Less Education START and University of Arkansas 

Having a Lower Social Economic Status START 

Failing to Achieve One’s Aspirations START 

Having Trouble in Romantic Relationships START 

Having Trouble in Platonic Relationships START 

Having Been Abused as an Adult START 

Being Distant From One’s Family START 

Being a Loner START 

Being Single START and University of Arkansas 

Being Younger START and University of Arkansas 

Having Military Experience University of Arkansas 

Being a Lone Actor START 

with engaging or attempting to engage 

in terrorism was examined on its own as 

opposed to in relationship with other risk 

factors. As a result, it is not possible to 

determine from the findings presented 

above how these potential risk factors 

interact with each other or whether 

particular combinations of risk factors have 

especially strong (or weak) relationships 

with engaging in terrorism. On the 

other hand, presenting the potential risk 

factors in this manner allows for more 

straightforward comparisons between 

findings from different studies. 

22 This and the following variable include cases in which individuals had histories of criminal activity but had not 

necessarily been convicted of these crimes. 

23 Further analysis revealed that having a purely nonviolent criminal history was not related to engaging or 

attempting to engage in terrorism. However, because the projects examining lone actors — as well as some of the 

risk assessment instruments and methods discussed later in the paper — use this broader variable, it is included 

in the table. 

24 The 465 individuals in the University of Arkansas project for whom data on both the legal and temporal/ 

geospatial aspects of their cases were available were included in this analysis. This represents the individuals 

indicted in approximately 20 percent of federal terrorism or terrorism-related cases between 1980 and 2015. The 

researchers found that the demographic characteristics of this sample were consistent with those of the total 

population of individuals indicted in federal terrorism or terrorism-related cases between 1980 and 2015 (Smith 

et al., 2016). 

25 This includes cases in which individuals had not necessarily received an official diagnosis of a mental disorder. 
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Potential risk factors associated with 

engaging or attempting to engage in 

terrorism among lone-actor extremists are 

displayed in Table 3 (Corner, Gill, & Mason, 

2015; Hamm & Spaaij, 2015a; Hamm & 

Spaaij, 2015b; Horgan et al., 2016). Cases in 

which the prevalence of the potential risk 

factor among lone-actor extremists could 

be compared with its prevalence among 

members of the general U.S. population26 

are listed first, followed by cases in which 

no comparison between individuals who did 

and did not engage or attempt to engage in 

terrorism is available. 

As can be seen from examining Tables 2 

and 3, having a criminal history, having 

mental health issues (or having received a 

diagnosis of schizophrenia or delusional 

disorder among lone actors), being 

unemployed, being single, being a loner 

or socially isolated, and having military 

experience were associated with a higher 

likelihood of engaging or attempting 

to engage in terrorism in research that 

included both group-based and lone-actor 

extremists, as well as in at least one study 

that included only lone-actor terrorists. 

These findings may be particularly 

important, as they were identified in studies 

that included different samples of terrorists, 

and they were based on at least one set 

of analyses that included a nonviolent 

comparison group. 

While the potential relationship between 

engaging or attempting to engage in 

terrorism and lacking employment or 

positive social support has been discussed 

in previous research (e.g., Schmid, 2013; 

Weine & Ahmed, 2012), the findings that 

having a previous criminal history and/ 

or mental health issues were potential 

risk factors for engaging or attempting 

to engage in terrorism seem striking. On 

the one hand, this may call into question 

the distinction that is often made between 

terrorists, who are thought to be driven by 

ideology, and “common” criminals, who 

are not (e.g., Pressman & Flockton, 2012). 

On the other hand, numerous studies have 

emphasized the psychological normality 

of terrorists (e.g., Post, Sprinzak, & Denny, 

2003; Sageman, 2005), although more 

recent research suggests that the reality 

may be more complex (Corner & Gill, 2015; 

Corner, Gill, & Mason, 2015; Gruenewald, 

Chermak, & Freilich, 2013). In addition, 

the finding that having military experience 

was associated with engaging or attempting 

to engage in terrorism is unexpected 

but possibly related to these individuals’ 

capabilities or perceived capabilities of 

carrying out an attack, which may play an 

important role in whether they attempt to 

do so (e.g., Lloyd & Dean, 2015; Pressman & 

Flockton, 2012; Willis et al., 2005). 

These findings also suggest possible 

avenues for prevention and intervention 

efforts as, with the exceptions of having 

a criminal record and having military 

experience, all of the potential risk factors 

identified are dynamic, meaning that 

they can change over time. Thus, in cases 

in which these potential risk factors are 

present and relevant, efforts to address 

individuals’ mental health issues, improve 

their employment prospects or situations, 

and/or provide them with stronger social 

support may contribute to prevention and 

intervention strategies. Further, as some of 

these types of efforts may also be relevant 

to individuals who engage in other types of 

concerning behaviors (e.g., gang activity, 

youth violence), adapting existing efforts 

focused on mitigating these behaviors may 

also be a possibility. 

26 Given that the vast majority of lone-actor terrorists were male, data on the prevalence of potential risk 

factors among members of the male U.S. population were used when available. In general, finding data on the 

prevalence of these various characteristics and behaviors in the general population was challenging, and efforts 

to systematically collect and catalog these types of data would benefit future research on terrorism and a range of 

other topics. 
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Potential Risk Factor 

(Prevalence Among Lone Actors)27 

Prevalence Among U.S. 

Population Identified By 

Being Male (100%) 

Being Male (99%) 

49%28 Indiana State University 

University of Massachusetts Lowell 

Being Single (80%) 

Being Single (81%) 

48% of males29 Indiana State University 

University of Massachusetts Lowell 

Being Unemployed (71%) 

Being Unemployed (38%) 

4% - 11% of males30 Indiana State University 

University of Massachusetts Lowell 

Having a Criminal Record (55%) 

Having Criminal Convictions (58%) 

<31%31 Indiana State University 

University of Massachusetts Lowell 

Living Alone (44%) 27%32 University of Massachusetts Lowell 

Having at Least a Bachelor’s Degree (40%) 32% of males33 University of Massachusetts Lowell 

Having Received a Diagnosis of Schizophrenia (13%) 

or Delusional Disorder (6%) 

<1%34 University of Massachusetts Lowell 

Having Military Experience (28%) 

Having Military Experience (32%) 

13% of males35 Indiana State University 

University of Massachusetts Lowell 

Having Personal & Political Grievances (80%) N/A Indiana State University 

Having an Enabler (67%) N/A Indiana State University 

Being Socially Isolated (51%) N/A University of Massachusetts Lowell 

Table 3: Potential Risk Factors Associated With Engaging or Attempting to Engage in Terrorism Among 

Lone-Actor Terrorists in the U.S. 

27 The percentages reported for the University of Massachusetts Lowell project represent the prevalence of the 

potential risk factors among all of the lone-actor terrorists included in the sample, whereas those reported for the 

Indiana State University project represent their prevalence among post-9/11 lone-actor terrorists who were not 

part of sting operations. 

28 According to the 2010 U.S. Census, 49.1 percent of the U.S. population is male: http://www.census.gov/prod/ 

cen2010/briefs/c2010br-03.pdf. 

29 According to the U.S. Census Bureau, as of 2013, 48 percent of the male U.S. population was never married, 

was divorced, or was widowed: http://factfinder.census.gov/faces/tableservices/jsf/pages/productview. 

xhtml?pid=ACS_14_5YR_DP02&src=pt /productview.xhtml?pid=ACS_10_3YR_S1201&prodType=table. 

30 According to the Bureau of Labor Statistics, the annual average unemployment rate for U.S. males between 

1975 and 2015 ranged from 4.1 percent to 10.5 percent: http://www.bls.gov/cps/cpsaat02.htm. 

31 According to the Brennan Center for Justice, as of 2015, between 70 million (22 percent) and 100 million (31 

percent) Americans had criminal records. Since not all of those who have records have been convicted of crimes, 

the percentage of Americans with criminal convictions would be less than 31 percent. Friedman, Matthew. “Just 

Facts: As Many Americans Have Criminal Records as College Diplomas.” New York University School of Law: 

Brennan Center for Justice. November 17, 2016. 

32 According to the U.S. Census Bureau, as of 2012, approximately 27 percent of U.S. households included only 

one person: http://www.census.gov/prod/2013pubs/p20-570.pdf. 

33 According to the U.S. Census Bureau, as of 2014, 31.9 percent of the male U.S. population 25 years or older held 

at least a bachelor’s degree: https://www.census.gov/hhes/socdemo/education/data/cps/2014/tables.html. 

34 According to the Centers for Disease Control and Prevention (Grohol, 2011), approximately 0.6 percent of the 

U.S. population will receive a diagnosis of schizophrenia during their lifetime and, according to the American 

Psychiatric Association (2013), approximately 0.2 percent of the U.S. population will receive a diagnosis of 

delusional disorder during their lifetime. 

35 As of 2014, approximately 13 percent of the male U.S. population was serving or had served in the military. 

Chalabi, Mona. “What Percentage of Americans Have Served in the Military?” FiveThirtyEight. March 19, 2015. 
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Fifteen additional risk factors emerged in 

analyses that compared both group-based 

and lone-actor extremists who did and did 

not engage in terrorism (P. James, personal 

communication, February 4, 2016; Jasko, 

LaFree, & Kruglanski, 2016; Sawyer, 2016; 

Smith et al., 2016). One of these risk factors 

was related to having a history of criminal 

violence; one was related to having been 

involved with a gang or delinquent peers; 

three were related to interactions with 

extremist individuals and/or ideologies 

(having a friend who is a terrorist, being 

a member of a terrorist group for an 

extended period of time, and having a deep 

commitment to an extremist ideology); four 

were related to individuals’ standing in 

professional, educational, and/or economic 

domains (having a sporadic work history, 

having less education, having a lower social 

economic status, and failing to achieve 

one’s aspirations); and four were related 

to potential problems in the personal 

relationship domain (having trouble in 

romantic relationships, having trouble in 

platonic relationships, being abused as an 

adult, and being distant from one’s family). 

In addition, being younger and being a lone 

actor (versus a group-based extremist) were 

also associated with a higher likelihood 

of engaging or attempting to engage in 

terrorism.36 

These findings once again suggest 

that, when relevant to a particular case, 

prevention and intervention efforts might 

benefit from focusing on improving 

individuals’ employment or educational 

prospects, thus increasing their ability to 

contribute to society in a positive manner 

as well as their ability to develop and 

sustain positive personal relationships with 

nonextremists and nondelinquent peers. 

They also suggest a role for addressing 

individuals’ extremist ideologies in 

prevention and intervention efforts. 

Finally, five potential risk factors were 

found in studies that focused on only 

lone-actor terrorists. These included living 

alone, which is likely related to being single 

and socially isolated, and having at least 

a bachelor’s degree, which may point to 

a potential difference in the relationship 

between education and engaging in 

terrorism among group-based versus loneactor terrorists. In addition, being male 

was associated with engaging or attempting 

to engage in terrorism among lone actors, 

which seems consistent with the finding 

that males are generally more likely than 

females to engage in homicide (U.S. 

Department of Justice, 2011).37 All three 

of these risk factors were more prevalent 

among lone-actor terrorists than among 

members of the general U.S. population. 

Finally, two other potential risk factors were 

common among lone-actor terrorists but 

were not identified based on comparisons 

with individuals who did not engage or 

attempt to engage in terrorism and, thus, 

the findings are more tentative. These 

included having personal and political 

grievances and having an enabler, for 

example, an individual who unwittingly 

assists in planning an attack or provides 

the inspiration for it (Hamm & Spaaij, 

2015a). Taken together, these findings 

36 There was also some evidence that having radicalized, at least in part, in prison was associated with engaging 

in terrorism in the analyses that included both group-based and lone actors, but because of the difficulty in 

assessing whether this was due, in part or in whole, to having a criminal history, having interactions with other 

terrorists, or something else, this potential risk factor is not included in the discussion. 

37 Although there was no overall relationship between being male and engaging or attempting to engage in 

terrorism in the analyses that included both group-based and lone-actor extremists, the University of Arkansas 

team found that being male was a risk factor for engaging or attempting to engage in terrorism among those who 

embraced certain groups or ideologies (e.g., those associated with al-Qa’ida) and that being female was a risk 

factor for engaging or attempting to engage in terrorism among those who embraced other groups or ideologies 

(e.g., environmental terrorism). 
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Potential Indicator (Prevalence Among Lone

Actors) Identified By 

Others Are Aware of Individual’s Grievance (80%) University of Massachusetts Lowell 

Others Are Aware of Individual’s Extremist Ideology (77%) University of Massachusetts Lowell 

Broadcasting Intent (76%) Indiana State University 

Expressing a Desire to Hurt Others (69%) University of Massachusetts Lowell 

Producing Public Statements About Extremist Ideology or University of Massachusetts Lowell 

Grievance (59%) 

Verbalizing Intent to Family or Friends (59%) University of Massachusetts Lowell 

Stockpiling Weapons (52%) University of Massachusetts Lowell 

Table 4: Potential Indicators Associated With Engaging or Attempting to Engage 

in Terrorism Among Lone-Actor Terrorists in the U.S. 

may underline the importance of paying 

attention to individuals’ social relationships, 

as well as understanding their grievances, in 

prevention and intervention programs. 

Potential Indicators 

To date, one potential indicator38 has been 

associated with engaging or attempting 

to engage in terrorism among both 

group-based and lone-actor extremists: 

specifically, individuals who attended 

more extremist group meetings were more 

likely to engage or attempt to engage in 

terrorism. This finding was based on an 

analysis that included a comparison group 

of extremists who did not engage or attempt 

to engage in terrorism (Smith et al., 2016).39 

Potential indicators associated with 

engaging or attempting to engage in 

terrorism among lone-actor extremists 

are displayed in Table 4 (Hamm & Spaaij, 

2015a; Hamm & Spaaij, 2015b; Horgan 

et al., 2016). None of these was based on 

analyses that compared individuals who 

did and did not engage in terrorism and, 

thus, these findings should be considered 

tentative. 

Although there was no overlap in the 

potential indicators identified in the 

research that included (1) both groupbased and lone-actor extremists and 

(2) only lone-actor terrorists, both of 

the projects that included only loneactor terrorists found that individuals 

who engaged or attempted to engage in 

terrorism tended to broadcast or verbalize 

their intent to do so beforehand. This 

seems to be an important finding as it 

implies that many lone-actor terrorists 

may provide advance warnings of their 

intentions to commit terrorism; and, if 

these warnings are taken seriously, there 

may be the potential to stop their attacks 

before they occur. It is also notable that 

one of the studies found that lone actors 

verbalized their intent specifically to family 

and friends, meaning that it may be those 

closest to these individuals who are best 

placed to observe this potential indicator. 

Other potential indicators included 

individuals actively conveying information 

about their grievances, extremist ideologies, 

or desires to hurt others, as well as 

others being aware of these lone actors’ 

grievances and extremist ideologies. Again, 

38 Again, an indicator may help to assess whether a behavior is actually occurring. 

39 The 465 individuals in the University of Arkansas project for whom data on both the legal and temporal/ 

geospatial aspects of their cases were available were included in this analysis. 
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this suggests that both the individuals 

themselves and those who surround them 

may provide valuable information that 

is potentially related to these individuals 

engaging or attempting to engage in 

terrorism. It is, however, important to keep 

in mind that these findings did not involve 

a comparison group and, as a consequence, 

it may be the case that there are more 

individuals who convey similar information 

about their grievances, extremist ideologies, 

and desires to hurt others but who do 

not go on to engage or attempt to engage 

in terrorism. Thus, while it arguably is 

important to be aware of these types of 

concerning statements, they should not be 

assumed to be either necessary or sufficient 

evidence that the individuals who produce 

them will follow their words with action. 

Finally, two of the potential indicators 

identified in NIJ-sponsored research 

involved activities possibly related to 

planning and preparing to engage in 

terrorism. One of these activities — 

attending more extremist group meetings 

— was identified when comparing groupbased and lone-actor extremists who did 

and did not engage or attempt to engage in 

terrorism. The other — stockpiling weapons 

— was found only among lone-actor 

terrorists and was not based on an analysis 

that included a comparison group. Because 

individuals may try to hide these behaviors 

from those who are close to them, it may be 

the case that others in the community are 

better positioned to observe them. 

Comparisons With Existing 

Risk Assessment Instruments 

and Methods 

Various instruments and methods have 

been developed to assess individuals’ 

risks of engaging in different types 

of illegal activity and violence, and 

three of these — the Extremist Risk 

Guidelines (Lloyd & Dean, 2015); the 

Violent Extremist Risk Assessment, 

Consultative Version 2 (Pressman & 

Flockton, 2012); and the Historical Clinical 

Risk Management-20, Version 3 (Douglas et 

al., 2013) — will be discussed below.40 While 

there are important differences between 

these instruments and methods, all three 

rely on structured professional judgment, 

meaning that they provide guidance on 

which risk factors or indicators41 to consider 

when making an assessment but do not rule 

out including others or dictate how these 

risk factors or indicators should be weighed 

(Monahan, 2012). Rather, it is assumed 

that each of these instruments or methods 

will be used by individuals who possess 

the expertise and experience to identify 

both the presence of risk factors and their 

relevance to a particular individual.42 

After briefly describing these instruments 

and methods, this paper examines 

similarities between the risk factors and/or 

indicators they highlight and the potential 

risk factors and indicators identified in 

NIJ-sponsored research. For the purpose 

of these comparisons, it should be kept in 

mind that it is often the case that specific 

40 The Multi-level Guidelines (MLG; Cook, Hart, & Kropp, 2013) have also been developed to assess the risk of 

group-based violence perpetrated by organized criminal networks, gangs, and terrorist groups, among others. 

Because the MLG was undergoing revision at the time of writing, the risk factors it includes will be discussed in a 

future paper. 

41 These instruments and methods also identify protective factors that should be considered in risk assessments. 

These will be discussed, along with relevant findings from NIJ-sponsored research, in a future paper. 

42 It is also assumed that identifying risk factors and indicators is only one component of a larger process that 

includes both risk assessment and management. This paper, however, focuses on this component. 
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risk factors or indicators included in the 

risk assessment instruments or methods 

were not analyzed in NIJ-sponsored 

research. One of the key reasons for this is 

that using the risk assessment instruments 

and methods discussed below often relies 

at least in part on the ability to conduct 

interviews with the individual who is 

being assessed. As the NIJ-sponsored 

studies generally analyzed information 

available in unclassified, secondary 

sources (e.g., newspaper reports, court 

records, biographies, government sources), 

information on certain risk factors (e.g., 

underlying motivations and thought 

patterns) was not always available. As a 

consequence, a lack of findings related 

to these factors does not mean that no 

relationships were found in NIJ-sponsored 

research but rather that the relationships 

were not tested. When no relationship or 

a negative relationship between a specific 

risk factor or indicator and engaging or 

attempting to engage in terrorism was 

found, this is indicated. 

Extremist Risk Guidelines 

(ERG 22+) 

The ERG 22+ methodology was developed 

by the United Kingdom’s National Offender 

Management Service (NOMS) in order 

to assess the risk posed by extremist 

offenders and facilitate the development of 

targeted interventions aimed at managing 

this risk (Lloyd & Dean, 2015).43 Based 

on casework with offenders and relevant 

social and behavioral science theory and 

research, NOMS identified 22 general 

risk factors for engaging in extremist 

crime (which includes but is not limited 

to terrorism). These factors are related to 

three dimensions: extremist engagement 

(the process by which individuals become 

involved with an extremist group, cause, 

or ideology); extremist intent (the mindset 

associated with the readiness to commit an 

extremist offense); and extremist capability 

(the ability to perform the offense) (Lloyd 

& Dean, 2015). Individuals are not assigned 

scores based on the ERG 22+; rather, the 

guidelines are used to suggest factors that 

may play a role in extremist offending. 

Table 5 lists the 22 factors included in 

the ERG 22+ and compares them with 

the potential risk factors and indicators 

identified in NIJ-sponsored research. 

As can be seen, the risk factors and/or 

indicators associated with attempting or 

engaging in terrorism identified in NIJsponsored research overlap to at least 

some degree with 10 of the risk factors for 

engaging in extremist crime included in 

the ERG 22+. These include risk factors 

related to expressing grievances; having 

family or friends who support extremist 

offending; having mental health issues; 

over-identifying with a group or cause; 

having attitudes that justify offending; 

supporting harmful means to achieving 

a specific end; supporting a harmful end; 

having individual knowledge, skills, and 

competencies; having access to networks, 

funding, and equipment; and having a 

criminal history. 

The amount of overlap is particularly 

interesting given that the ERG 22+ includes 

risk factors for engaging in extremist 

crimes that do not necessarily reach the 

threshold of engaging or attempting to 

engage in terrorism, which was the focus 

of the NIJ-sponsored findings discussed 

above. (In fact, in many cases, individuals 

who engaged in extremist crimes that 

did not reach this threshold were used as 

the comparison group in NIJ-sponsored 

analyses.) Future research will be needed 

to determine whether there are particular 

risk factors and indicators included in NIJsponsored research that can distinguish 

between those who do and do not engage in 

nonviolent extremist crimes. 

43 While initially developed for use with those already convicted of extremist offenses, the ERG 22+ has also 

informed community approaches to preventing individuals from engaging in these offenses (Lloyd & Dean, 

2015). 
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ERG 22+ Risk Factors Risk Factors Identified in NIJ-Sponsored Research 

(Relevant Dimensions Are in Parentheses) (Relevant Indicators Are in Parentheses) 

Need to Redress Injustice and Express Grievance Having Personal and Political Grievances — Lone Actors Only 

(Engagement) (Others Aware of Grievance, Producing Public Statements About 

Extremist Ideology or Grievance — Both Among Lone Actors Only) 

Need to Defend Against Threat (Engagement) 

Need for Identity, Meaning, Belonging (Engagement) 

Need for Status (Engagement) 

Need for Excitement, Comradeship, Adventure 

(Engagement) 

Need for Dominance (Engagement) 

Susceptibility to Indoctrination (Engagement) 

Political/Moral Motivation (Engagement) 

Opportunistic Involvement (Engagement) 

Family or Friends Support Extremist Offending Having a Terrorist Friend* 

(Engagement) 

Transitional Periods (Engagement) 

Group Influence and Control (Engagement) 

Mental Health (Engagement) Having Psychological Issues 

Having Received a Diagnosis of Schizophrenia or Delusional 

Disorder — Lone Actors Only 

Over-Identification With Group or Cause (Intent) Having a Deep Commitment to an Extremist Ideology 

(Attending More Extremist Group Meetings) 

Us and Them Thinking (Intent) 

Dehumanization of the Enemy (Intent) 

Attitudes That Justify Offending (Intent) Having a Deep Commitment to an Extremist Ideology 

(Others Aware of Extremist Ideology, Producing Public Statements About 

Extremist Ideology or Grievance — Both Among Lone Actors Only) 

Harmful Means to an End (Intent) (Verbalizing Intent to Family and Friends, Broadcasting Intent — Both 

Among Lone Actors Only) 

Harmful End Objectives (Intent) (Expressing a Desire to Hurt Others — Lone Actors Only) 

Individual Knowledge, Skills, and Competencies (Capability) Having Military Experience 

Access to Networks, Funding, Equipment (Capability) Having an Enabler — Lone Actors Only 

(Stockpiling Weapons — Lone Actors Only) 

Criminal History (Capability) Having a Criminal History 

* Potential risk factors and indicators that are in bold were identified based on comparisons with nonviolent extremists or the general U.S. 

population. (Potential risk factors and indicators identified in NIJ-sponsored research may be associated with more than one risk factor 

included in the ERG 22+.) 

Table 5: Comparison Between the Risk Factors Included in the ERG 22+ and the Potential Risk Factors 

and Indicators Identified in NIJ-Sponsored Research 
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Violent Extremist Risk 

Assessment, Consultative Version 

2 (VERA 2) 

The VERA 2 is focused specifically on 

assessing the risk of violent political 

extremism (Pressman & Flockton, 2012). 

The 25 risk factors and indicators included 

in the tool are related to four areas — 

beliefs and attitudes; context and intent; 

history and capability; and commitment and 

motivation — and were identified based on 

research in the areas of radicalization and 

terrorism, collaboration with operational 

personnel, and feedback from experts in 

law enforcement, corrections, and forensic 

psychology. While the VERA 2 is generally 

intended for use after an individual has 

been convicted of a terrorism-related 

offense, its developers state that it can 

also be used to support investigative 

procedures (Pressman & Flockton, 2012). 

In all cases, however, it is recommended 

as a supplement, not replacement, for 

existing risk assessment instruments and 

procedures. Table 6 lists the 25 risk factors 

and indicators included in the VERA 2 

and compares them with the potential risk 

factors and indicators identified in NIJsponsored research. 

As can be seen in Table 6, the risk 

factors and/or indicators associated with 

attempting or engaging in terrorism 

identified in NIJ-sponsored research 

overlap to at least some degree with nine 

of the risk factors and indicators included 

in the VERA 2. Similar to the comparison 

with the ERG 22+, these include risk 

factors and indicators related to having 

grievances; having personal contact with 

terrorists (family or friends supporting 

extremist offending in the ERG 22+); 

having a commitment to a group or 

group ideology (over-identifying with a 

group or cause in the ERG 22+); having 

attitudes that justify violence (offending 

in the ERG 22+); expressing the intent 

to act violently or in a harmful manner; 

having tactical, paramilitary, or explosives 

training (individual knowledge, skills, 

and competencies in the ERG 22+); and 

having access to funds, resources, and 

organizational skills (networks, funding, 

and equipment in the ERG 22+). Unlike 

the ERG 22+, the VERA 2 also includes 

risk factors associated with having family 

members or friends involved in violent 

action and having a prior criminal history 

of violence. Interestingly, the risk factor 

related to having mental health issues is 

not included in the VERA 2, although it 

was identified as a potential risk factor 

for engaging or attempting to engage in 

terrorism in NIJ-sponsored research and is 

included in the ERG 22+. 

The Historical Clinical Risk 

Management-20, Version 3 

(HCR-20v3) 

The HCR-20v3 is a method used to assess 

the risk of violence among adults and is 

most frequently applied within correctional, 

forensic, and general or civil psychiatric 

settings (Douglas et al., 2013). Since the 

first version of the method was released 

in 1995, numerous studies have been 

conducted to evaluate its validity, and 

the most recent version of the HCR-20, 

released in 2013, incorporates revisions 

based on additional research on the risk 

factors associated with violence as well as 

advances in the field of risk assessment 

and management. The 20 items identified 

in the HCR-20v3 include historical factors 

(related to individuals’ past behaviors and 

experiences), clinical factors (related to 

their psychosocial functioning, mental 

health, and behaviors in the more recent 

past, e.g., in the past six months), and 

risk management factors (related to areas 

of future functioning and adjustment) 

(Douglas et al., 2013). The method provides 

guidelines for evaluating both the presence 

of these 20 risk factors and their relevance 

to a particular individual. 
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Table 6: Comparison Between the Risk Factors and Indicators Included in the VERA 2 and the Potential 

Risk Factors and Indicators Identified in NIJ-Sponsored Research 

Risk Factors Identified in NIJ-Sponsored 

Research (Relevant Indicators Are in 

Parentheses) 

VERA 2 Risk Factors and Indicators 

(Relevant Areas Are in Parentheses) 

Commitment to Ideology Justifying Violence (Beliefs & Attitudes) Having a Deep Commitment to an Extremist Ideology* 

(Others Aware of Extremist Ideology, Producing Public 

Statements About Extremist Ideology or Grievance — 

Both Among Lone Actors Only) 

Victim of Injustice and Grievances (Beliefs & Attitudes) Having Personal and Political Grievances — Lone 

Actors Only 

(Others Aware of Grievance — Lone Actors Only) 

Dehumanization/Demonization of Identified Targets of Injustice (Beliefs & 

Attitudes) 

Rejection of Democratic Society and Values (Beliefs & Attitudes) 

Feelings of Hate, Frustration, Persecution, Alienation (Beliefs & Attitudes) 

Hostility to National Collective Identity (Beliefs & Attitudes) 

Lack of Empathy, Understanding Outside of Own Group (Beliefs & Attitudes) 

Seeker, Consumer, Developer of Extremist Materials (Context & Intent) 

Identification of Target (Person, Group, Place) in Response to Perceived 

Injustice (Context & Intent) 

Personal Contact With Violent Extremists (Context & Intent) Being a Member of a Terrorist Group for an Extended 

Period of Time 

Having a Terrorist Friend 

(Attending More Extremist Group Meetings) 

Anger and Expressed Intent to Act Violently (Context & Intent) (Expressing a Desire to Hurt Others, Verbalizing Intent 

to Family and Friends, Broadcasting Intent — All Among 

Lone Actors Only) 

Expressed Desire to Die for Cause or Martyrdom (Context & Intent) 

Expressed Intent to Plan, Prepare Violent Action (Context & Intent) 

Susceptible to Influence, Authority, Indoctrination (Context & Intent) 

Early Exposure to Pro-Violence Militant Ideology (History & Capability) 

Network (Family, Friends) Involved in Violent Action (History & Capability) Having a Terrorist Friend 

Prior Criminal History of Violence (History & Capability) Having a History of Criminal Violence 

Tactical, Paramilitary, Explosives Training (History & Capability) Having Military Experience 

Extremist Ideological Training (History & Capability) 

Access to Funds, Resources, Organizational Skills (History & Capability) Having an Enabler — Lone Actors Only 

(Stockpiling Weapons — Lone Actors Only) 

Glorification of Violent Action (Commitment & Motivation) 

Driven by Criminal Opportunism (Commitment & Motivation) 

* Potential risk factors and indicators that are in bold were identified based on comparisons with nonviolent extremists or the general U.S. 

population. (Potential risk factors and indicators identified in NIJ-sponsored research may be associated with more than one risk factor or 

indicator included in the VERA 2.) 
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Risk Factors Identified in NIJ-Sponsored 

VERA 2 Risk Factors and Indicators Research (Relevant Indicators Are in 

(Relevant Areas Are in Parentheses) Parentheses) 

Commitment to Group, Group Ideology (Commitment & Motivation) Having a Deep Commitment to an Extremist 

Ideology 

(Attending More Extremist Group Meetings) 

Driven by Moral Imperative, Moral Superiority (Commitment & Motivation) 

Driven by Excitement, Adventure (Commitment & Motivation) 

Table 6: Comparison Between the Risk Factors and Indicators Included in the VERA 2 and the Potential 

Risk Factors and Indicators Identified in NIJ-Sponsored Research (continued) 

Table 7 lists the 20 risk factors for violence 

included in the HCR-20v3 and compares 

them with the potential risk factors and 

indicators associated with engaging 

or attempting to engage in terrorism 

in NIJ-sponsored research. When the 

exact timeframe in the past during 

which individuals experienced or were 

exposed to specific risk factors was not 

clear, these risk factors were considered 

historical factors as opposed to clinical 

factors. In two cases, however, risk factors 

that individuals were experiencing or 

exposed to at the time they engaged or 

attempted to engage in terrorism were 

considered future risk factors based on the 

assumption that (1) risk assessments would 

have been conducted before individuals 

attempted to engage in terrorism and 

(2) those assessing the potential risk of 

violence would have viewed these factors 

as potentially impacting the individuals’ 

future functioning and adjustment. 

As can be seen in Table 7, the risk 

factors and/or indicators associated with 

attempting or engaging in terrorism 

identified in NIJ-sponsored research 

overlap to at least some degree with nine 

of the risk factors for engaging in violence 

included in the HCR-20v3. Similar to the 

comparison with the ERG 22+, these include 

risk factors related to having a major 

mental disorder (mental health issues in 

the ERG 22+) and a history of engaging in 

nonviolent antisocial behavior (crime in the 

ERG 22+). Similar to the comparison with 

the VERA 2, these include the risk factor 

related to having a history of problems 

with violence (a prior criminal history of 

violence in the VERA 2). Similar to the 

comparisons with both the ERG 22+ and 

VERA 2, these include risk factors related 

to having attitudes that justify violence 

(offending in the ERG 22+). 

What is perhaps most striking, however, is 

that five of the risk factors included in the 

HCR-20v3 that were also found in research 

sponsored by NIJ were not included in 

either the ERG 22+ or VERA 2, both of 

which were developed to assess the risk 

of criminal behavior among extremists. 

Specifically, risk factors related to having 

a history of problems with relationships, 

employment, and traumatic experiences, 

as well as having future problems with 

one’s living situation and personal support, 

were all associated with the likelihood 

of engaging in violence in the HCR-20v3 

and among terrorists examined in NIJsponsored research. 

Overall Comparison 

While the previous three subsections 

examine how the potential risk factors 

and indicators identified in NIJ-sponsored 
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HCR-20v3 Risk Factors 

Risk Factors Identified in NIJ-Sponsored Research 

(Relevant Indicators Are in Parentheses) 

History of Problems With Violence Having a History of Criminal Violence* 

History of Problems With Other Antisocial Behavior Having Been Involved With a Gang or Delinquent Peers 

History of Problems With Relationships Having Trouble in Romantic Relationships 

Having Trouble in Platonic Relationships 

Having Been Abused as an Adult 

Being Distant From One’s Family 

Being a Loner/Socially Isolated 

Having Been Involved With a Gang or Delinquent Peers 

Having a Terrorist Friend 

Having an Enabler 

History of Problems With Employment Having a Sporadic Work History 

Being Unemployed 

History of Problems With Substance Use44 

History of Problems With Major Mental Disorder (psychotic, 

major mood, or other major disorder) 

Having Received a Diagnosis of Schizophrenia or Delusional Disorder — 

Lone Actors Only 

History of Problems With Personality Disorder 

History of Problems With Traumatic Experiences Having Been Abused as an Adult 

History of Problems With Violent Attitudes Having a Deep Commitment to an Extremist Ideology 

(Expressing a Desire to Hurt Others, Others Aware of Extremist Ideology, 

Producing Public Statements About Extremist Ideology or Grievance, 

Verbalizing Intent to Family and Friends, Broadcasting Intent — All Among 

Lone Actors Only) 

History of Problems With Treatment or Supervision Response 

Recent Problems With Insight (mental disorder, violence risk, 

need for treatment) 

Recent Problems With Violent Ideation or Intent 

Recent Problems With Symptoms of Major Mental Disorder 

(psychotic, major mood, or other major disorder) 

Recent Problems With Instability (affective, behavioral, 

cognitive) 

Recent Problems With Treatment or Supervision Response 

Future Problems With Professional Services and Plans 

Future Problems With Living Situation Being Unemployed 

Future Problems With Personal Support Being Distant From One’s Family 

Being a Loner/Socially Isolated 

Having a Terrorist Friend 

Having an Enabler 

Future Problems With Treatment or Supervision Response 

Future Problems With Stress or Coping 

Table 7: Comparison Between the Risk Factors Identified in the HCR-20v3 and the Potential Risk Factors 

and Indicators Identified in NIJ-Sponsored Research 

* Potential risk factors and indicators that are in bold were identified based on comparisons with nonviolent extremists or the general U.S. 

population. (Potential risk factors and indicators identified in NIJ-sponsored research may be associated with more than one risk factor 

included in the HCR-20v3.) 

44 Research conducted by START found that having a substance use problem was not associated with an increased likelihood that individuals 

engaged or attempted to engage in terrorism (P. James, personal communication, February 4, 2016). 
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Table 8: Comparison Between Categories of Potential Risk Factors and Indicators 

Identified in NIJ-Sponsored Research and Those Included in Existing Risk 

Assessment Instruments and Methods 

Categories of Potential Risk Factors and Indicators 

Category ERG 22+ VERA 2 HCR-20v3 

Past History of Violence X X 

Past History of Criminal Activity and Delinquency X X 

Extremist Associates X X 

Extremist Ideology and/or Grievances X X 

Intent to Commit Violence X X 

Skills and Resources for Committing Violence X X 

Mental Health Issues X X 

Trouble in Professional, Educational, and 

Economic Domains X

Trouble in Relationship Domains X 

Specific Demographic Characteristics 

research map onto the risk factors and 

indicators included in the ERG 22+, VERA 

2, and HCR-20v3, this subsection does 

the reverse; i.e., it maps the types of risk 

factors and indicators included in these 

instruments and methods onto those 

identified in NIJ-sponsored research. In this 

manner, it is possible to identify categories 

of potential risk factors or indicators that 

are not included in these instruments 

and methods but that may need to be 

considered when assessing the risk that an 

individual may engage or attempt to engage 

in terrorism. 

As the first step in this process, the 

potential risk factors and indicators 

identified in NIJ-sponsored research 

were grouped into 10 broader categories. 

These included having (1) a past history 

of violence; (2) a past history of criminal 

activity or delinquency; (3) extremist 

associates; (4) an extremist ideology 

and/or grievances; (5) the intent to 

commit violence; (6) skills and resources 

for committing violence; (7) mental 

health issues; (8) trouble in professional, 

educational, and economic domains; (9) 

trouble in relationship domains; and (10) 

specific demographic characteristics.45 

Table 8 lists the 10 categories of potential 

risk factors and indicators identified in 

NIJ-sponsored research and indicates 

whether they are included in the three 

risk assessment instruments and methods 

discussed above.46 

As can be seen in Table 8, nine out of 

the 10 categories of potential risk factors 

and indicators associated with engaging 

or attempting to engage in terrorism in 

NIJ-sponsored research are included in at 

least one of the existing risk assessment 

instruments or methods, the exception 

being specific demographic characteristics. 

Six of the categories are included in the 

HCR-20v3, which was developed to assess 

the risk of individuals engaging in general 

45 The individual risk factors and indicators associated with each of these broader categories are identified in the 

Appendix. 

46 In some cases, an individual risk factor or indicator associated with a particular category may map onto one of 

the risk factors included in the ERG 22+, VERA 2, or HCR-20v3, but the category itself may not be present in the 

instrument or method. For example, “having a violent extremist friend” mapped onto the “history of problems 

with relationships” and “future problems with personal support” risk factors in the HCR-20v3 — both of which 

include references to relationships with individuals who exert a negative or antisocial influence — but a category 

of risk factors and indicators associated specifically with having “extremist associates” was not included in the 

HCR-20v3. 
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violence, and five are included in each of 

the instruments and methods developed 

specifically to assess the risk of individuals 

engaging in extremist crime and/or 

violence (the ERG 22+ and VERA 2). 

While there is clear overlap in the 

categories of risk factors and indicators 

identified in NIJ-sponsored research and 

those included in the ERG 22+, VERA 2, and 

HCR-20v3, there are also some distinctions 

based on whether the instruments and 

methods are focused on assessing the risk 

of general violence versus extremist crime 

and/or terrorism. On the one hand, only 

the HCR-20v3 includes risk factors and 

indicators related to having (1) trouble in 

professional, educational, and economic 

domains and (2) trouble in relationship 

domains, both of which were found to be 

associated with engaging or attempting 

to engage in terrorism in NIJ-sponsored 

research. On the other hand, only the ERG 

22+ and VERA 2 include risk factors and 

indicators related to having (1) extremist 

associates, (2) an extremist ideology and/ 

or grievances, and (3) skills and resources 

for committing violence, which were also 

found to be associated with engaging or 

attempting to engage in terrorism in NIJsponsored research. Overall, this suggests 

that while many of the risk factors and 

indicators associated with engaging or 

attempting to engage in terrorism may be 

similar to those associated with engaging 

in general violence, other risk factors and 

indicators may be unique to those involved 

in extremist-related crime and terrorism. 

Overview of Findings and 

Next Steps 

Before summarizing the overall findings 

from the NIJ-sponsored research discussed 

in this paper, it is important to reiterate 

some of its limitations. For example, it is 

likely the case that some of the individuals 

included in the analyses conducted by these 

four NIJ-sponsored project teams overlap, 

and that, because of this, similarities 

between the projects’ findings may be 

due in part to the fact that they analyzed 

some of the same individuals. There are 

also issues related to missing data and 

the possibility that some of the findings 

presented would not be the same if data 

were available for all of the individuals 

included in the projects. Further, even 

when data were available for individuals, 

it may be the case that some of the 

information collected was not accurate. 

Likewise, assessing whether individuals 

have experienced or been exposed to 

certain risk factors may require subjective 

judgments on the part of researchers, 

and it is possible that different research 

teams might reach different conclusions. 

Finally, once again, it is important to note 

that not all individuals who experience or 

are exposed to risk factors — or exhibit 

indicators — associated with engaging or 

attempting to engage in terrorism go on 

to do so. Radicalization to terrorism is a 

complex process, and research that analyzes 

samples of terrorists can help to identify 

characteristics, experiences, and behaviors 

that are generally associated with engaging 

or attempting to engage in terrorism, 

but further investigation is required to 

determine whether these characteristics, 

experiences, and behaviors are relevant in 

the case of a particular individual. 

Still, keeping these limitations in mind, 

the research sponsored by NIJ has made 

a strong contribution by identifying 

numerous risk factors and indicators 

potentially associated with engaging 

or attempting to engage in terrorism. 

Importantly, in many cases these findings 

emerged from analyses that compared 

extremists who engaged or attempted 

to engage in terrorism with extremists 

who did not (or with the general U.S. 

population); and, as a consequence, there 

is evidence that these potential risk factors 

and indicators may help practitioners to 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 21 

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distinguish between individuals who are 

more and less likely to attempt or conduct 

a terrorist attack, as well as provide 

some foundation for the development of 

prevention and intervention programs. 

Several potential risk factors that may 

increase the likelihood that an individual 

will engage or attempt to engage in 

terrorism were found in research that 

included group-based and lone-actor 

extremists, as well as in at least one study 

that included only lone-actor terrorists. 

These risk factors (all of which were 

identified based on comparisons between 

individuals who did and did not engage 

in these behaviors) include having a 

criminal history, having mental health 

issues (or having received a diagnosis 

of schizophrenia or delusional disorder 

among lone actors), being unemployed, 

being single, being a loner/socially isolated, 

and having military experience. These 

findings suggest that, in cases in which 

these potential risk factors are present and 

relevant, efforts to address mental health 

issues, improve employment prospects or 

situations, and/or provide stronger social 

support may contribute to prevention and 

intervention strategies. 

Potential risk factors that were found only 

in analyses that included both group-based 

and lone-actor extremists — and which 

also included nonviolent comparison 

groups — related to having a history of 

criminal violence; having been involved 

with a gang or delinquent peers; being 

younger; being a lone actor (versus a groupbased extremist); having interactions with 

extremist individuals and/or ideologies; 

having a lower standing in professional, 

educational, and/or economic domains; 

and having problems in the personal 

relationship domain. These again suggest 

that prevention efforts might benefit 

from focusing on improving individuals’ 

employment or educational prospects 

and/or their ability to develop and sustain 

positive personal relationships with 

nonextremists and nondelinquent peers. 

They also suggest a role for addressing 

individuals’ extremist ideologies in 

prevention and intervention efforts. 

Finally, a few potential risk factors were 

found only in analyses of lone-actor 

terrorists. Being male, living alone, and 

having at least a bachelor’s degree were all 

more prevalent among lone-actor terrorists 

than among the general U.S. population. 

Two other potential risk factors — having 

personal and political grievances and 

having an enabler — were not identified 

based on comparisons with individuals who 

did not engage or attempt to engage in 

terrorism and, thus, are considered more 

tentative. Taken together, these findings 

again seem to underline the importance 

of paying attention to individuals’ social 

relationships in prevention and intervention 

programs. They also suggest that 

understanding individuals’ personal and 

ideological grievances may be important. 

Compared with risk factors, fewer potential 

indicators associated with engaging or 

attempting to engage in terrorism were 

identified in NIJ-sponsored research — and 

only one of these was based on an analysis 

that included a nonviolent comparison 

group. The findings suggest that those 

who are closest to an individual may be 

well positioned to observe some of the 

potential warning signs that he or she may 

be moving toward terrorism. Specifically, 

both of the projects that included only 

lone-actor terrorists found that individuals 

who engaged or attempted to engage in 

terrorism tended to broadcast or verbalize 

their intent to do so beforehand, and in 

one analysis they tended to verbalize their 

intent specifically to family and friends. 

Other potential indicators included 

individuals (and specifically lone actors) 

actively conveying information about their 

grievances, extremist ideologies, or desires 

to hurt others, as well as others being 

aware of their grievances and extremist 

ideologies. It is, however, important to keep 

in mind that these findings did not involve 

a comparison group; and, as a consequence, 
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it may be the case that there are more 

individuals who convey similar information 

about their grievances, extremist ideologies, 

and desires to hurt others but who do not 

go on to engage or attempt to engage in 

terrorism. 

Finally, two other potential indicators 

associated with engaging or attempting to 

engage in terrorism included attending 

more extremist group meetings (based 

on an analysis that included a nonviolent 

comparison group) and stockpiling 

weapons. In both of these cases, individuals 

may try to hide these behaviors from those 

who are close to them, and thus, it may be 

that others in the community are better 

positioned to observe them. 

This paper also found substantial overlap 

between the risk factors and indicators 

identified in NIJ-sponsored research and 

risk factors and/or indicators included 

in existing instruments and methods 

developed to assess the risk of individuals 

engaging in extremist crime (the ERG 

22+), extremist violence (the VERA 2), 

and general violence (the HCR-20v3). 

Specifically, potential risk factors and/ 

or indicators identified in NIJ-sponsored 

research overlapped to at least some degree 

with 10 of the risk factors for engaging 

in extremist crime included in the ERG 

22+, nine of the risk factors and indicators 

associated with engaging in extremist 

violence included in the VERA 2, and nine 

of the risk factors for engaging in general 

violence included in the HCR-20v3. 

Looking more broadly at the general 

categories of potential risk factors and 

indicators identified in NIJ-sponsored 

research, nine out of 10 were included in 

at least one of the existing risk assessment 

instruments or methods. These included 

categories of potential risk factors and 

indicators related to having (1) a past 

history of violence; (2) a past history 

of criminal activity or delinquency; (3) 

extremist associates; (4) an extremist 

ideology and/or grievances; (5) the intent 

to commit violence; (6) skills and resources 

for committing violence; (7) mental 

health issues; (8) trouble in professional, 

educational, and economic domains; and 

(9) trouble in relationship domains. There 

were, however, some distinctions in the 

categories included based on whether 

the instruments and methods focused 

on assessing the risk of general violence 

(the HCR-20v3) or extremist crime and/or 

violence (the ERG 22+ and VERA 2). On 

the one hand, only the HCR-20v3 includes 

risk factors and indicators related to having 

(1) trouble in professional, educational, 

and economic domains and (2) trouble in 

relationship domains, both of which were 

found to be associated with engaging or 

attempting to engage in terrorism in NIJsponsored research. On the other hand, 

only the ERG 22+ and VERA 2 include risk 

factors and indicators related to having 

(1) extremist associates, (2) an extremist 

ideology and/or grievances, and (3) skills 

and resources for committing violence, 

which were also identified as important 

categories of risk factors and indicators in 

NIJ-sponsored research. 

These findings suggest that many of 

the potential risk factors and indicators 

associated with engaging in terrorism may 

not be that different from those associated 

with engaging in violence more generally 

and, thus, programs developed to mitigate 

the risk of violence among perpetrators who 

are not motivated by extremist ideologies 

may have relevance to those who are. They 

also suggest that some risk factors and 

indicators may indeed be different for those 

who commit terrorism, such as those related 

to having extremist associates, an extremist 

ideology and/or grievances, and skills 

and resources for committing violence. 

Consequently, all of these factors may need 

to be considered when assessing the risk 

that an individual will engage or attempt to 

engage in terrorism and when developing 

prevention and intervention efforts. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 23 

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Overall, the research sponsored to date by 

NIJ has provided important insights into 

the potential risk factors and indicators 

associated with engaging or attempting to 

engage in terrorism, but much work still 

remains to be done. First, although some of 

the projects reviewed in this paper involved 

comparing individuals who engaged or 

attempted to engage in terrorism with 

those who did not, other projects did not 

include nonviolent comparison groups. 

As including such groups is a necessary 

condition for identifying valid risk factors 

and indicators, as well as protective factors, 

conducting more research that includes 

relevant comparison groups should be a 

priority. Further, systematically collecting 

and cataloguing population-level data 

related to potential risk factors, protective 

factors, and indicators would aid in this 

endeavor. 

Second, although the analyses discussed 

in this paper included terrorists who 

embraced a variety of ideologies, additional 

research needs to be conducted to 

examine whether particular risk factors 

and indicators associated with engaging 

or attempting to engage in violence vary 

by extremist ideology. There is already 

some evidence that this may be the case 

for gender, with the University of Arkansas 

team finding that being male was a risk 

factor for engaging or attempting to engage 

in terrorism among those who embraced 

certain ideologies (e.g., those associated 

with al-Qa’ida) and that being female was 

a risk factor for engaging or attempting 

to engage in terrorism among those who 

embraced others (e.g., environmental 

terrorism). Future research will need 

to explore additional risk factors and 

indicators in order to inform whether 

different risk assessment instruments 

and methods, as well as prevention and 

intervention programs, may need to 

be developed for individuals who hold 

different extremist ideologies. 

Third, in both of the studies that included 

only lone-actor terrorists, there was 

evidence that potential risk factors and 

indicators associated with engaging 

in violence may change over time. For 

example, when comparing lone-actor 

terrorists and solo mass murderers who 

committed attacks between 1990 and 

2005 with those who committed attacks 

between 2006 and 2013, the University 

of Massachusetts Lowell team found 

significant differences in these individuals’ 

characteristics, experiences, and behaviors, 

depending on the timeframe in which 

they acted (Horgan et al., 2016). These 

differences included the tendency for fewer 

individuals who committed attacks between 

2006 and 2013 to have military experience, 

verbalize their intent to family and friends, 

express a desire to hurt others, or have 

others aware of their grievances. Although 

the inclusion of solo mass murderers in 

this analysis makes it difficult to assess how 

these findings specifically relate to loneactor terrorists, they do highlight the need 

for further research on how risk factors 

and indicators associated with engaging or 

attempting to engage in terrorism may vary 

based on the time period during which the 

behavior occurs. They also suggest that any 

risk assessment instruments and methods 

will need to be periodically reviewed and 

updated, based on the latest data. 

Fourth, while this paper focused on risk 

factors and indicators associated with 

engaging or attempting to engage in 

terrorism, it is also important to examine 

risk factors and indicators associated with 

engaging in other behaviors of concern 

(e.g., extremist crime more generally, such 

as that assessed by the ERG 22+). Further, 

research on risk factors and indicators 

associated with noncriminal behaviors 

such as adopting an extremist belief system 

or joining an extremist group may be of 

value to practitioners focused on early 

intervention efforts. Although some of the 
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risk factors and indicators associated with 

these different behaviors may overlap, it 

is likely that others do not and that it may 

be necessary to develop risk assessment 

instruments and methods focused on 

specific behaviors of concern and/or the 

various roles that individuals can play in 

terrorist organizations (e.g., Nesser, 2006; 

Victoroff, 2005). 

Fifth, examining how risk factors interact 

and combine with each other — as well as 

with protective factors — is an important 

area for future research. For example, NIJsponsored research conducted by Brandeis 

University47 has found that specific “triads,” 

or sets of three sequential indicators, were 

present in significant percentages of the 

timelines of U.S. homegrown terrorists 

inspired by or affiliated with al-Qa’ida 

(Klausen, 2016). Reports from the 

individual research projects discussed in 

this paper also begin to take on the task of 

looking at how risk factors and indicators 

combine, but more work, particularly in 

the area of protective factors, remains to be 

done. Further, in doing this work, it will be 

important to ensure that the implications of 

the findings for developing risk assessment 

tools and methods, as well as prevention 

and intervention programs, are clearly 

articulated. 

Sixth, many of the risk factors and 

indicators included in the existing risk 

assessment instruments and methods 

discussed above were not examined in 

NIJ-sponsored research. Although in some 

cases these risk factors and indicators 

may not have been analyzed because of a 

different focus on the part of the research 

teams, in some cases, they could not be 

analyzed because the teams did not have 

access to the type of data necessary to 

do so (e.g., interview data). Important 

findings can come from research based on 

secondary analysis, as this paper has sought 

to demonstrate. However, only when these 

findings can be supplemented with those 

based on interviews and other primary 

sources will it be possible to develop truly 

evidence-based risk assessment instruments 

and methods. 

47 The projects “The Role of Social Networks in the Evolution of Al Qaeda-inspired Violent Extremism in the 

United States, 1993-2013” (NIJ Award #2012-ZA-BX-0006) and “Prisoner Recollections: The Role of Internet 

Use and Real-Life Networks in the Early Radicalization of Islamist Terrorist Offenders” (NIJ Award #2013-ZABX-0005) are led by PI Jytte Klausen and conducted by a project team that includes Selene Campion, Zachary 

Herman, Rosanne Libretti, Nathan Needle, Giang Nguyen, Adrienne Roach, Eliane Tschaen Barbieri, and Aaron 

Zelin. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 25 

National Institute of Justice | NIJ.ojp.gov

References 

American Psychiatric Association (2013). Diagnostic and statistical manual of mental disorders 

(5th ed.). Washington, DC: American Psychiatric Publishing. 

Centers for Disease Control and Prevention (2011). Mental illness surveillance among adults 

in the United States. Morbidity and Mortality Weekly Report, 60 (Supplement), 1–30. 

Cook, A., Hart, S., & Kropp, P. (2013). Multi-level guidelines for the assessment and 

management of group-based violence. Burnaby, Canada: Mental Health, Law, and Policy 

Institute, Simon Fraser University. 

Corner, E., & Gill, P. (2015). A false dichotomy? Mental illness and lone-actor terrorism. Law 

and Human Behavior, 39, 23–34. 

Corner, E., Gill, P., & Mason, O. (2015). Mental health disorders and the terrorist: A 

research note probing selection effects and disorder prevalence. Studies in Conflict & 

Terrorism, doi:10.1080/1057610X.2015.1120099. 

Douglas, K., et al. (2013). HCR-20V3: Assessing risk of violence – User guide. Burnaby, Canada: 

Mental Health, Law, and Policy Institute, Simon Fraser University. 

Grohol, J. (2011). CDC statistics: Mental illness in the US. Psych Central. 

Gruenewald, J., Chermak, S., & Freilich, J. (2013). Distinguishing “loner” attacks from other 

domestic extremist violence. Criminology & Public Policy, 12, 65–91. 

Hamm, M., & Spaaij, R. (2015a). Lone wolf terrorism in America: Using knowledge of 

radicalization pathways to forge prevention strategies. Available at: https://www.ncjrs.gov/ 

pdffiles1/nij/grants/248691.pdf. 

Hamm, M., & Spaaij, R. (2015b). Lone wolf terrorism in America: Radicalization and 

prevention. In Seri Irazola (Chair), Lone wolves and lone actors: Patterns of radicalization. 

Breakout session conducted at the conference Radicalization and Violent Extremism: 

Lessons Learned From Canada, the United Kingdom, and the US, Arlington, Va., July 2015. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 26 

National Institute of Justice | NIJ.ojp.gov

Horgan, J., et al. (2016). Across the universe? A comparative analysis of violent behavior and 

radicalization across three offender types with implications for criminal justice training and education. 

Available at: https://www.ncjrs.gov/pdffiles1/nij/grants/249937.pdf. 

Jasko, K., LaFree, G., & Kruglanski, A. (2016). Quest for significance and violent extremism: 

The case of domestic radicalization. Unpublished manuscript, University of Maryland. 

Klausen, J., et al. (2016). A behavioral study of “homegrown” radicalization trajectories. 

Studies in Conflict & Terrorism, 39, 67–83, doi:10.1080/1057610X.2015.1099995. 

LaFree, G. (2015) In Pete Simi (Chair), Politically-motivated radicalization to violent 

extremism. Breakout session conducted at the conference Radicalization and Violent 

Extremism: Lessons Learned from Canada, the United Kingdom, and the US, Arlington, 

Va., July 2015. 

Lloyd, M., & Dean, C. (2015). The development of structured guidelines for assessing risk in 

extremist offenders. Journal of Threat Assessment and Management, 2, 40–52. 

Monahan, J. (2012). The individual risk assessment of terrorism. Psychology, Public Policy, 

and Law, 18, 167–205. 

Nesser, P. (2006). Jihad in Europe: Recruitment for terrorist cells in Europe. In L. Bokhari 

et al. (Eds.), Paths to global jihad: Radicalisation and recruitment to terror networks (9–21). 

Kjeller, Norway: Norwegian Defence Research Establishment. 

Post, J., Sprinzak, E., & Denny, L. (2003). The terrorists in their own words: Interviews with 

35 incarcerated Middle Eastern terrorists. Terrorism and Political Violence, 15, 171–184. 

Pressman, E., & Flockton, J. (2012). Calibrating risk for violent political extremists and 

terrorists: The VERA 2 structured assessment. The British Journal of Forensic Practice, 14, 

237–251. 

Sageman, M. (2005). Understanding terror networks. Philadelphia: University of Pennsylvania 

Press. 

Sawyer, J.P. (2016). What makes them do it? Individual-level predictors of terrorism. 

Unpublished manuscript, University of Maryland. 

Scalora, M., et al. (2003). Risk factors for approach behavior toward the U.S. Congress. 

Journal of Threat Assessment, 2, 35–55. 

Schmid, A. (2013). Radicalisation, de-radicalisation, counter-radicalisation: A conceptual 

discussion and literature review. ICCT research paper. The Hague: The International 

Center for Counter-Terrorism. 

Smith, B., et al. (2016). Identity and framing theory, precursor activity, and the radicalization 

process. Available at: https://www.ncjrs.gov/pdffiles1/nij/grants/249673.pdf. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 27 

National Institute of Justice | NIJ.ojp.gov

U.S. Department of Justice (2011). Homicide trends in the United States: 1980–2008. Available 

at: https://www.bjs.gov/content/pub/pdf/htus8008.pdf. 

Victoroff, J. (2005). The mind of the terrorist: A review and critique of psychological 

approaches. Journal of Conflict Resolution, 49, 3–42. 

Weine, S., & Ahmed, O. (2012). Building resilience to violent extremism among SomaliAmericans in Minneapolis-St. Paul. Final report to Human Factors/Behavioral Sciences 

Division, Science and Technology Directorate, U.S. Department of Homeland Security. 

College Park, MD: START. 

Willis, H., et al. (2005). Estimating terrorism risk. Santa Monica, CA: RAND Corporation. 
Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States 28 

National Institute of Justice | NIJ.ojp.gov

Categories of Potential Risk Factors and Indicators Identified in NIJ-Sponsored Research 

Potential Risk Factors and Indicators Identified in NIJ-Sponsored Research 

Categories Individual Risk Factors 

(Relevant Indicators Are in Parentheses) 

Past History of Violence Having a History of Criminal Violence* 

Past History of Criminal Activity and Delinquency Having a Criminal History 

Having Been Involved With a Gang or Delinquent Peers 

Extremist Associates Having a Terrorist Friend 

Being a Member of a Terrorist Group for an Extended Period of Time 

(Attending More Extremist Group Meetings) 

Extremist Ideology and/or Grievances Having a Deep Commitment to an Extremist Ideology 

Having Personal & Political Grievances — All Among Lone Actors Only 

(Others are Aware of Extremist Ideology, Others are Aware of Grievances, Producing 

Public Statements About Extremist Ideology or Grievances — All Among Lone Actors Only) 

Intent to Commit Violence (Broadcasting Intent, Verbalizing Intent to Family or Friends, Expressing a Desire to 

Hurt Others — All Among Lone Actors Only) 

Skills and Resources for Committing Violence Having Military Experience 

Having an Enabler — Among Lone Actors Only 

(Stockpiling Weapons — Among Lone Actors Only) 

Mental Health Issues Having Psychological Issues 

Having Received a Diagnosis of Schizophrenia or Delusional Disorder — 

Among Lone Actors Only 

Trouble in Professional, Educational, and 

Economic Domains 

Being Unemployed 

Having a Sporadic Work History 

Having Less Education48 

Having a Lower Social Economic Status 

Failing to Achieve One’s Aspirations 

Trouble in Relationship Domains Having Trouble in Romantic Relationships 

Having Trouble in Platonic Relationships 

Having Been Abused as an Adult 

Being Distant From One’s Family 

Being a Loner or Socially Isolated 

Specific Demographic Characteristics Being Single 

Being Younger 

Living Alone — Among Lone Actors Only 

Being Male — Among Lone Actors Only 

* Potential risk factors and indicators that are in bold were identified based on comparisons with nonviolent extremists or the general U.S. population. 

48 Lone-actor terrorists were actually found to have achieved a higher level of education than the average American male. 

Appendix