NCJ 250171

U.S. Department of Justice

Office of Justice Programs

National Institute of Justice

National Institute of Justice

How Radicalization to Terrorism Occurs 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.
U.S. Department of Justice

Office of Justice Programs

810 Seventh St. N.W.

Washington, DC 20531

David B. Muhlhausen, Ph.D.

Director, National Institute of Justice

This and other publications and products of the National Institute of Justice can be found at:

National Institute of Justice

Strengthen Science • Advance Justice

NIJ.ojp.gov

Office of Justice Programs

Building Solutions • Supporting Communities • Advancing Justice

OJP.gov

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 

Assistance; the Bureau of Justice Statistics; the Office for Victims of Crime; the Office of Juvenile Justice and Delinquency 

Prevention; and the Office of Sex Offender Sentencing, Monitoring, Apprehending, Registering, and Tracking.

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.
How Radicalization to Terrorism Occurs in the United States i

National Institute of Justice | NIJ.ojp.gov

Executive Summary 

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

Terrorism program has sponsored research on how radicalization to terrorism occurs in the 

United States in order to support prevention and intervention efforts. These projects have 

taken a variety of approaches to examining the process of radicalization to terrorism, but in 

spite of this there is substantial overlap in their findings, which collectively provide evidence 

of the importance of several facilitators of radicalization and the need to take into account 

how this process unfolds within individuals over time. 

Facilitators of Radicalization to Terrorism

Terrorist belief systems or narratives

Identity processes

Activities that demonstrate commitment to a terrorist group or cause

Connections with terrorists in one’s offline social network

Connections with terrorists via the internet and/or social media

Group dynamics

Grievances

Triggering events

At the individual level, the radicalization process often involves embracing a terrorist belief 

system or narrative that identifies particular others or groups as “enemies” and justifies 

engaging in violence against them. Individuals may also begin to identify themselves as 

terrorists, as well as to engage in activities that highlight their commitments to their new 

beliefs, identities, and/or others who hold them. It is, however, important to note that while 

these beliefs and behaviors may facilitate the movement to terrorism, this outcome is not 

inevitable. Those close to these individuals may become aware of the changes that their 

friends and family members are undergoing and attempt to address them or seek help from 

others who can. An important implication of this is that trusted information and resources 

need to be available to assist in this effort. Another is that prevention and intervention 

efforts may benefit from addressing beliefs that justify violence and helping individuals to 

develop identities in which these beliefs are not central. 
How Radicalization to Terrorism Occurs in the United States ii

National Institute of Justice | NIJ.ojp.gov

NIJ-sponsored research also highlights 

the social nature of radicalization to 

terrorism and the roles that connections 

with terrorists (online and/or offline) and 

group dynamics may play in this process. 

As individuals’ relationships with others 

who support terrorism become stronger, 

they may begin separating themselves 

from those who do not share these beliefs, 

thus becoming increasingly isolated from 

people who might challenge their views. 

This suggests that dramatic changes in the 

people with whom an individual associates, 

or increasing insularity among existing 

groups of friends, may be causes for 

concern. It also suggests that intervention 

efforts may need to take into account both 

the individuals and those with whom they 

interact, as well as potentially facilitate 

establishing or re-establishing their 

relationships with nonextremists.

Finally, there is evidence that events in the 

larger community and societal contexts may 

also facilitate radicalization to terrorism. For 

example, individuals may experience real 

or perceived grievances — both personal 

and political — that may help to fuel their 

movement toward terrorism. Further, in some 

cases, specific triggering events associated 

with these grievances may accelerate 

this movement. Thus, understanding 

these grievances and triggering events, 

as well as helping individuals to develop 

constructive ways to address them, may 

be important components of programs 

developed to prevent and intervene in the 

radicalization process. 

In addition to identifying several common 

facilitators of radicalization to terrorism, 

the research sponsored by NIJ provides 

empirical evidence that individuals’ 

processes of radicalization to terrorism 

may vary by the extremist ideologies 

and narratives they embrace, the time 

periods in which they radicalize, the 

groups or movements they join (or do 

not join, in the case of lone actors), and/

or their individual characteristics and 

experiences. Thus, while community 

members and practitioners can benefit 

from the types of evidence-based guidance 

provided by this research, it will continue 

to remain important that they take into 

consideration the specific characteristics 

and experiences of the individuals about 

whom they are concerned.
How Radicalization to Terrorism Occurs in the United States 1

National Institute of Justice | NIJ.ojp.gov

How Radicalization to Terrorism 

Occurs in the United States: What 

Research Sponsored by the National 

Institute of Justice Tells Us

Introduction

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

Terrorism program has sponsored research on how radicalization to terrorism1 occurs in 

the United States in order to support prevention and intervention efforts.2 In doing so, it 

has funded top researchers from around the country — and the world — to examine the 

process of radicalization using both well-established and cutting-edge social and behavioral 

science methods. These teams have studied a wide range of groups and individuals who 

support and commit ideologically motivated violence to further political, social, or religious 

goals, including Islamist terrorists but also those associated with anti-government, anticapitalist, nativist, and other political and social terrorist movements (often referred to as 

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

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

of radicalization to terrorism and the need to consider how this process evolves within 

individuals over time have begun to emerge. This paper summarizes them and identifies 

some of their implications for community members and practitioners conducting prevention 

and intervention efforts. 

1

 There are many 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.

2

 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).
How Radicalization to Terrorism Occurs in the United States 2

National Institute of Justice | NIJ.ojp.gov

The paper begins by examining the 

findings from four NIJ-sponsored 

research projects that have taken different 

approaches to examining the process of 

radicalization to terrorism. After discussing 

the samples used in the projects and 

some general limitations that need to 

be considered when interpreting their 

findings, the paper describes the research 

conducted by Brandeis University to 

test whether an existing framework of 

radicalization was supported by the data 

the team collected on U.S. homegrown 

offenders convicted of terrorist acts inspired 

by or affiliated with al-Qa’ida.3 It then 

turns to a project conducted by researchers 

from Indiana State University and Victoria 

University on lone-wolf terrorism in the 

U.S. and the model of radicalization to 

terrorism that emerged from their analysis.4

Next, it describes research conducted by 

the National Consortium for the Study 

of Terrorism and Responses to Terrorism 

(START), based at the University of 

Maryland, to examine the radicalization 

processes and trajectories of U.S. terrorists 

motivated by a range of ideologies.5 Finally, 

it discusses research led by the University 

of Arkansas that tested whether two 

theoretical perspectives on radicalization to 

terrorism were supported in data collected 

on individuals and groups indicted for 

terrorism or terrorism-related activities in 

U.S. federal courts.6

After providing a brief scientific overview 

of these efforts, the paper highlights the 

collective insights they provide into the 

process of radicalization to terrorism. It 

also discusses some of the implications of 

these findings for community members and 

practitioners conducting prevention and 

intervention efforts.

Scientific Overview of Four 

NIJ-Sponsored Projects That 

Examine Radicalization to 

Terrorism

The four projects highlighted in this 

section focus on different yet overlapping 

subsets of violent and nonviolent extremists 

active in the U.S. Table 1 includes some 

key characteristics of the individuals and 

groups included in these projects.

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 an al-Qa’ida-inspired lone actor 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 

3

 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-ZA-BX0005) are led by Principal Investigator (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.

4

 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 (Indiana State University) and conducted 

in collaboration with Ramon Spaaij (Victoria University, Australia).

5

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

6

 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).
How Radicalization to Terrorism Occurs in the United States 3

National Institute of Justice | NIJ.ojp.gov

be ruled out, the differences between the 

specific categories of terrorists included in 

the various projects (e.g., only those inspired 

by or affiliated with al-Qa’ida in the Brandeis 

University project, lone actors who had varied 

ideologies in the Indiana University project) 

make it less likely that any similarities in their 

findings are due exclusively to an overlap in 

the individuals analyzed.

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. In the projects 

discussed below, this often resulted 

in individuals for whom data were not 

available being excluded from an analysis, 

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

Project Team

Lone 

Actors/ 

Group 

Actors 

Ideologies 

Embraced Description

Brandeis University Both Al-Qa’idainspiredThis analysis includes 135 homegrown terrorism 

offenders inspired by or affiliated with al-Qa’ida.7 The 

time period covered is from 1993 to 2015.

Indiana State 

University/Victoria 

University

Lone 

Actors

Varied This analysis includes all 98 individuals who engaged 

in terrorism but acted alone, did not belong to a group, 

and were not directed by others.8

 The time period 

covered is from 1940 to 2013.

START Both Varied This project randomly sampled 1,475 individuals from 

the larger population that met its inclusion criteria.9

 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 The quantitative analysis conducted for this project 

includes 465 individuals indicted for violent or 

nonviolent terrorism-related activities in U.S. federal 

court between 1980 and 2013.10 The qualitative analysis 

includes 43 organizations indicted for violent terrorismrelated activities between 1980 and 2013.

7

 These offenders include only individuals who were born and grew up in the U.S. or who arrived in the U.S. 

before the age of 16 and, thus, were assumed to have radicalized in the U.S. The Western Jihadism Project from 

which this sample was drawn includes nearly 400 individuals who meet these criteria. Only the 135 individuals for 

whom sufficient data on their radicalization trajectories could be collected are included in the analysis discussed 

in this paper.

8

 The project defined “lone-wolf terrorism” as political violence perpetrated by individuals who act alone, who 

do not belong to an organized terrorist group or network, who act without the direct influence of a leader or 

hierarchy, and whose tactics and methods are conceived of and directed by individuals without any direct outside 

command or direction.

9

 The project includes 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 of ideologically motivated violent offenses.

10 At the time the project was completed, the research team was tracking 1,062 federal terrorism-related cases 

involving 2,195 individuals. Only those individuals for whom data on both the legal and geospatial/temporal 

components of their cases had been collected are included in the quantitative analysis.
How Radicalization to Terrorism Occurs in the United States 4

National Institute of Justice | NIJ.ojp.gov

and in certain cases, this led to findings 

that were based on only a portion of the 

sample. As a consequence, it is not always 

possible to assess whether the findings 

would be the same if data were available 

for all of the individuals included in the 

project. Further, even when data were 

available for all individuals, 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 

engaged in certain activities, such as 

seeking out a new authority figure or 

joining a clique, 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 an activity that one team 

found important to analyze was not viewed 

as important by other teams and, thus, was 

not analyzed.

Finally, with one exception — the 

quantitative analysis conducted by the 

University of Arkansas — none of the 

findings highlighted below were based on 

comparisons between individuals who did 

and did not engage in terrorism-related 

activity. Because of this, while the behaviors 

and processes the teams identify may be 

considered characteristic of the extremists 

they studied, it is not clear whether they 

differ from the behaviors and processes 

that characterize similar extremists who do 

not engage in terrorism-related activities. 

As a consequence, these findings provide 

information on the process of radicalization 

to violence, but they cannot identify the 

causes of terrorism.

With these caveats in mind, the rest 

of this section describes the findings 

from four NIJ-sponsored projects that 

examined different subsets of terrorists 

and used a range of different methods to 

analyze the data that they collected. In 

spite of their differences, the common 

goal of these efforts was arriving at a 

better understanding of the process of 

radicalization to terrorism and how it may 

unfold over time.

Testing an Existing Framework of 

Radicalization to Terrorism

Against the background of the growing 

literature on Islamist terrorism in the West 

(e.g., Berger, 2011; Klausen, 2012; Nesser, 

2014; Sullivan, Freilich, & Chermak, 2014), 

one of the goals of the completed NIJsponsored research conducted by Brandeis 

University involved testing whether the 

framework of radicalization developed by the 

New York Police Department (NYPD; Silber 

& Bhatt, 2007) was supported by the more 

extensive data that the project has collected 

on U.S. homegrown terrorism offenders 

inspired by or affiliated with al-Qa’ida 

(Klausen, 2015b; Klausen et al., 2015). The 

researchers were particularly interested 

in examining this framework because it 

provided an opportunity for them to study 

how behavioral indicators evolve — and are 

related to each other — as the radicalization 

process progresses. 

The NYPD outlined a four-stage process 

of radicalization (Silber & Bhatt, 2007). 

The first stage, “pre-radicalization,” is the 

period before any radicalization occurs. 

At this point, individuals have not been 

exposed to an extremist belief system 

and lead relatively normal lives, although 

there may be aspects of their experiences 

and environments (e.g., isolation, 

discrimination) that potentially make 

them more vulnerable to radicalization. 

The second stage, “self-identification,” 

involves individuals’ introduction to 

and eventual embrace of an extremist 
How Radicalization to Terrorism Occurs in the United States 5

National Institute of Justice | NIJ.ojp.gov

belief system. The authors argue that 

this is sometimes prefaced by individuals 

experiencing economic, social, political, 

or personal crises and that exposure 

to extremist views and narratives may 

occur through individuals’ existing social 

networks, including their friends and 

family, or through movements promoting 

these beliefs. During the third stage, 

“indoctrination,” individuals’ extremist 

beliefs intensify, and they view violence as 

necessary to supporting these beliefs. At 

this stage individuals tend to join groups 

that embrace the same terrorist views and 

disassociate from individuals who do not. 

During the final stage, “jihadization,” the 

terrorist group or movement’s goals take 

complete precedence for individuals, and 

the final steps are taken toward violence 

as individuals train, acquire weapons, and 

ultimately attempt to carry out attacks.

To test the framework, the Brandeis 

University team created a typology of 

detectable behavioral indicators associated 

with each of the four stages included in the 

NYPD framework: (1) pre-radicalization, 

(2) detachment (referred to as “selfidentification” in the original framework), 

(3) peer immersion (referred to as 

“indoctrination” in the original framework), 

and (4) planning and execution of violent 

action (referred to as “jihadization” in the 

original framework). These indicators, 

displayed in Table 2 below, were selected 

based on the broader terrorism literature, 

as well as the team’s previous research. A 

pilot study was conducted to determine 

whether it was possible to identify data 

relevant to the indicators using open-source 

data, and adjustments to the indicators were 

made as needed.

Table 2: Behavioral Indicators of Stage Progression in Radicalization Trajectories (Klausen, 2016, 9)

Stage Pre-Radicalization Detachment

Peer Immersion and 

Training

Planning and 

Execution of Violent 

Action

Description Searching behavior indicative 

of cognitive opening

Detachment from 

previous life, e.g., by 

spending inordinate 

amounts of time with 

online extremist peers

Leaving home to become 

closer to a peer group of likeminded individuals

Attempting or enacting 

violent action — or joining 

a terrorist group abroad or 

attempting to join a group

This could 

include:

Expressions of disillusionment 

with world affairs or 

with religious or political 

authorities

Behavior indicative of a 

personal crisis in response 

to personal events, e.g., a 

family crisis, drug addiction, 

incarceration, or being 

arrested

Seeking out information 

in venues outside the 

individual’s established social 

milieu, either online or in real 

life, from new authority figures

Actively seeking to get 

closer to new authority 

figures or proselytizing 

online or in real life

Experiencing a revelation 

or making changes in 

lifestyle such as dropping 

out of school or work

Picking fights with 

the local mosque or 

teachers, colleagues, 

and family — or 

otherwise trying to 

convince others to 

change by starting a blog 

or a website

Attempting to go abroad to join 

an organization or a network 

to live as prescribed by the 

ideology

Behavior indicative of a desire 

to permanently join the militant 

community, e.g., by finding a 

spouse (or spouses) through 

the extremist community

Seeking out ways to 

demonstrate commitment to 

the new ideological community 

and its mission, e.g., by 

acquiring practical training in 

the use of firearms or other 

skills considered important to 

the mission of the extremist 

community

Actively supporting 

another person carrying 

out violent action on behalf 

of the ideology

Issuing threats online, in 

real life, or in other ways

Joining a foreign terrorist 

organization or taking 

practical steps to carry 

out an attack, e.g., by 

acquiring materials needed 

to fabricate a bomb or 

purchasing a firearm
How Radicalization to Terrorism Occurs in the United States 6

National Institute of Justice | NIJ.ojp.gov

For its analysis, the team collected 

biographical data on 135 U.S. homegrown 

terrorism offenders inspired by or affiliated 

with al-Qa’ida between 1993 and 2015 from 

court documents, online communications 

posted by the individuals, media interviews, 

and investigations conducted by the 

U.S. government and news media. The 

researchers then examined whether these 

individuals exhibited any of the behavioral 

indicators they had identified, and if so, 

when the behaviors occurred.11 Once this 

process was completed, the team used a 

computer algorithm to determine whether 

the data supported the sequence of stages 

outlined in the NYPD framework.

The analysis offered support for the 

sequence of stages outlined by NYPD. As 

expected, the types of indicators associated 

with the pre-radicalization stage (e.g., 

personal crises, traumatic events) were 

generally observed early in offenders’ 

timelines. In addition, peer immersion 

indicators (e.g., training, seeking likeminded extremists) tended to occur 

often and in sequence. At the same time, 

indicators associated with the detachment 

stage (e.g., proselytizing online or offline) 

tended to occur across multiple stages, 

and a few indicators chosen for the model 

performed poorly. For example, having an 

epiphany, or a revelation that one has been 

called by a higher power to take a particular 

action, was rarely experienced and, when 

it was, it did not occur in a specific stage 

(Klausen, 2015b).

The researchers also found that specific 

“triads,” or sets of three sequential 

indicators that span at least two stages in 

the radicalization process, were present in 

significant percentages of the offenders’ 

timelines (Klausen, 2015b). For example, 

the combination of any pre-radicalization 

indicator followed by offline peer immersion 

and then expressing a desire for action 

occurred in 47 percent of timelines. Similarly, 

the combination of any detachment indicator 

followed by offline peer immersion and then 

expressing a desire for action occurred in 34 

percent of timelines.

Finally, the team was interested in 

exploring the time it took individuals 

to move from their initial exploration 

of extremist ideas to engaging in a 

terrorist offense and whether there were 

differences based on when an individual 

radicalized. Comparing individuals who 

radicalized before 2010 with individuals 

who radicalized in or after 2010, they 

discovered that those who radicalized 

before 2010 typically took five to six 

years to move from initial exploration 

to a terrorist offense while those who 

radicalized during or after 2010 typically 

took 19 months (Klausen, 2015b). While 

further research is necessary in order 

to identify the reasons for this apparent 

acceleration of the radicalization process, 

the team found that this difference was 

not exclusively related to an increase in 

online radicalization; in fact, of the four 

individuals who radicalized in six months 

or less (all in 2014 or 2015), none had 

radicalized solely or even primarily online. 

Further, to test whether the acceleration 

was due to more proactive law enforcement 

efforts, the team compared individuals 

who were arrested in FBI-directed sting 

operations with those who were not. 

Although the radicalization trajectories 

of offenders who were arrested in sting 

operations were shorter, this did 

not account for the entire difference 

(Klausen, 2015a).

In summary, the Brandeis team found that 

many, but not all, aspects of the NYPD 

11 If the team was unable to find information about a behavioral indicator for a particular individual, the 

individual was included in the analysis and the behavior was assumed not to have occurred. 
How Radicalization to Terrorism Occurs in the United States 7

National Institute of Justice | NIJ.ojp.gov

framework were supported by their data 

on U.S. homegrown terrorism offenders 

inspired by or affiliated with al-Qa’ida.12

They also found that it was possible to 

identify sequences of indicators that were 

common among numerous offenders. 

Assuming the promise of these early findings 

continues to be supported, it would seem that 

examining the types of behavioral indicators 

identified by the team — and how these 

indicators evolve over time — can provide 

practitioners with a better understanding 

of how radicalization occurs among U.S. 

homegrown individuals inspired by or 

affiliated with al-Qa’ida, as well as possible 

ways of identifying where someone may be 

in this process.

Developing a New Model of 

Radicalization to Terrorism Among 

Lone Wolves in the U.S.

Rather than testing an existing model, the 

project conducted by Indiana State University 

and Victoria University developed a new 

model of radicalization to terrorism based on 

analysis of 98 cases of lone-wolf terrorism that 

occurred in the U.S. between 1940 and 2013 

(Hamm & Spaaij, 2015). For each of these 

cases, the researchers gathered information 

from a range of sources, including but 

not limited to biographies and memoirs, 

journalistic sources, government reports, and 

court documents. They then examined all 

of the information they had collected and 

entered data on the specific characteristics, 

experiences, and behaviors of each lone-wolf 

terrorist into a database. Their analysis of this 

database, as well as key case studies, produced 

several findings relevant to the process by 

which lone wolves in the U.S. radicalize to 

terrorism and serve to complement other 

research conducted on lone-actor terrorism in 

the U.S. and Europe (e.g., Gill, 2015; Horgan 

et al., 2016; Feve & Bjornsgaard, 2016). The 

model they developed is presented in Figure 1 

and described below. 

12 This may be due to issues with the framework, issues with the data the team collected, or both.

Figure 1: Model of Radicalization to Terrorism Among Lone Wolves in the U.S. 

(Hamm & Spaaij, 2015, 26)

Enabler

Broadcasting 

Intent

Terrorism

Triggering

Event

Personal and 

Political 

Grievances Affinity with Online 

Sympathizers or 

Extremist Group
How Radicalization to Terrorism Occurs in the United States 8

National Institute of Justice | NIJ.ojp.gov

The researchers found evidence that 

lone-wolf terrorists frequently combined 

personal grievances (i.e., perceptions that 

they had been personally wronged) with 

political grievances (i.e., perceptions that a 

government entity or other political actor 

had committed an injustice). Both personal 

and political grievances were present in 80 

percent of the cases they examined, and there 

was no difference in their prevalence when 

comparing lone wolves who attacked before 

and after September 11, 2001.13 They argue 

that this fusion of grievances is a signature of 

lone-wolf terrorism and usually happens at 

the beginning of the radicalization process.

As the process of radicalization to terrorism 

progressed, the researchers found that lone 

wolves frequently displayed an affinity with 

online sympathizers or an extremist group, 

and thus, the assumption that they do not 

communicate with others is false. The team 

does, however, note that the percentage 

of lone wolves who had connections with 

specific extremist groups (versus more diffuse 

networks of extremist sympathizers) declined 

after 9/11: Only 42 percent of the lone wolves 

who conducted attacks after 9/11 were found 

to have an affinity with an extremist group, 

compared with 63 percent of the lone wolves 

who conducted attacks before 9/11. The 

researchers attribute much of this change to 

evolving communication technologies (e.g., 

the internet, social media).

The data also indicated that lone wolves were 

often enabled by others who unwittingly 

assisted them in planning attacks (e.g., by 

purchasing materials they did not know 

would be used to conduct attacks) and/or 

who provided them with inspiration. This 

was the case among 57 percent of those who 

conducted attacks before 9/11 and 67 percent 

of those who conducted attacks after 9/11. 

Along the same lines, the researchers found 

evidence of copycat attacks in one-third of the 

cases that they examined.

The research showed that as lone wolves 

continued to radicalize, they tended to 

broadcast their intent to conduct attacks, e.g., 

by communicating with outsiders through 

spoken statements, threats, letters, manifestos 

and videotaped proclamations. Specifically, 

84 percent of the lone wolves who conducted 

attacks before 9/11 and 76 percent who 

conducted attacks after 9/11 either explicitly 

referred to an upcoming attack or implied 

that an attack was imminent (sometimes more 

than once). 

Finally, the researchers found that lone wolves 

frequently experienced personal and/or 

political triggering events (e.g., losing a job, 

witnessing government actions they opposed) 

that served as catalysts for their attacks. There 

was evidence that 84 percent of those who 

conducted attacks before 9/11 and 71 percent 

of those who conducted attacks after 9/11 

experienced a triggering event. They argue 

that triggering events may constitute one of 

the turning points in life that separate the 

past from the present and allow individuals 

to develop new relationships that support or 

encourage radicalization to terrorism.

In summary, based on the research 

conducted by Indiana State University 

and Victoria University, there seem to be 

signatures of the process of radicalization 

to terrorism among lone-wolf terrorists, 

at least those who conduct attacks in the 

U.S. From an intervention perspective, a 

few points are of particular interest. First, 

lone-wolf terrorists did communicate 

and interact with others, meaning that 

there were other individuals aware that 

they were radicalizing. Second and along 

13 When comparing individuals who attacked before and after 9/11, the researchers did not include the 15 

post-9/11 cases that were the result of law enforcement sting operations involving confidential informants or 

undercover agents. They argue that these are not authentic cases of lone-wolf terrorism because they involved 

more than one person. They did, however, include these cases in the database because sting operations have 

become a major counterterrorism strategy since 9/11.
How Radicalization to Terrorism Occurs in the United States 9

National Institute of Justice | NIJ.ojp.gov

the same lines, they tended to broadcast 

their intent to commit attacks, which may 

allow for interventions before attacks 

occur. Finally, many lone-wolf terrorists 

experienced triggering events, highlighting 

the importance of considering the role that 

political and personal events may play in 

individuals’ trajectories to violence.

Examining Radicalization 

Processes and Trajectories 

Through Quantitative and 

Qualitative Analysis

In their ongoing NIJ-sponsored research, 

a team from START is conducting both 

quantitative and qualitative analysis to 

examine the radicalization processes and 

trajectories of individuals who radicalized 

in the U.S. between 1965 and 2013 

(Jensen, 2015; LaFree, 2015). To do this, 

the researchers have created the Profiles 

of Individual Radicalization in the United 

States (PIRUS) database, which includes 

information on the characteristics, 

experiences, and behaviors of a sample of 

more than 900 terrorists and almost 600 

nonviolent extremists, and they are in the 

process of developing life-course narratives 

for 110 individuals in the database. These 

efforts will complement several existing 

open-source databases that include 

information on terrorism in the U.S. and 

internationally (e.g., Canadian Network 

for Research on Terrorism, Security, and 

Society, 2016; Freilich et al., 2014; LaFree 

& Dugan, 2007). The information used 

to develop both the PIRUS database 

and the life-course narratives comes 

from unclassified, secondary sources 

(e.g., newspaper reports, court records, 

biographies, government sources).

Several of the variables in the PIRUS 

database provide information on the 

radicalization processes of the individuals 

it includes. For example, the team found 

evidence that approximately 60 percent 

of individuals for whom relevant data 

were available14 experienced some kind 

of grievance, whether against the U.S. 

government, against a foreign government, 

or in reaction to a specific political 

event. The latter type of grievance was 

particularly common among Islamist 

terrorists, with the radicalization processes 

of approximately 72 percent of them 

affected by a political event (LaFree, 2015). 

There was also evidence that the internet 

played at least some role in individuals’ 

radicalization in just under one-half of the 

cases and that being a member of a clique 

(i.e., a close-knit, insular, and exclusive 

group of people) contributed to the 

radicalization of approximately 42 percent 

of individuals (Jensen, 2016). Both of these 

processes were also more common among 

Islamist terrorists. 

While the qualitative component of the 

project will eventually involve analyzing 

the life-course narratives of 110 individuals 

chosen to represent a diversity of ideologies 

and behaviors, to date the researchers have 

focused on examining how the narratives of 

15 individuals map onto four hypothetical 

radicalization pathways they have developed 

(Jenson, 2015). Each of these pathways 

incorporates mechanisms associated with 

various existing theories of radicalization 

to terrorism, including but not limited to 

the quest for personal significance (e.g., 

Kruglanski et al., 2009), social identity 

theory (e.g., Hogg & Adelman, 2013), 

and rational choice theory (e.g., Taylor & 

Quayle, 1994). 

To do this the team has used processtracing, a method that examines the 

links between hypothesized causes and 

effects. A description of the hypothesized 

pathways and the findings from this 

14 All of the percentages reported exclude individuals for whom relevant data were not available. 
How Radicalization to Terrorism Occurs in the United States 10

National Institute of Justice | NIJ.ojp.gov

preliminary analysis are in Table 3 below. 

Two individuals’ narratives did not map on 

to any of the pathways.

While additional research remains to be 

conducted, preliminary qualitative findings 

from the START project indicate that 

there are multiple radicalization pathways. 

Further, in their quantitative analysis of 

the PIRUS database, the researchers found 

evidence that grievances, the internet, 

and small group dynamics play roles in 

radicalization, although the prevalence of 

these mechanisms may vary by ideology. 

Testing Two Theoretical 

Perspectives on Radicalization to 

Terrorism

In their completed NIJ-sponsored research, 

a team led by the University of Arkansas 

tested whether role identity theory and 

framing theory help to explain how 

individuals and groups radicalize to 

terrorism (Smith et al., 2016). In doing 

so, the researchers built on previous 

research conducted using the American 

Terrorism Study (ATS) database (e.g., 

Smith & Damphousse, 2009; Smith, 

Damphousse, & Roberts, 2006; Smith, 

Roberts, & Damphousse, 2013) and 

focused not on testing a complete model 

of the radicalization process but rather on 

testing what they viewed to be a crucial 

component of it: the construction of a 

terrorist identity. This process is outlined 

in Figure 2 on page 11.15

Several concepts displayed in the figure are 

central to understanding the analyses the 

team conducted. The first two are identity 

salience and identity pervasiveness — both 

of which are based on the premise that 

everyone has multiple identities (e.g., as a 

parent, teacher, musician). If an identity 

is salient, it is more likely to be brought to 

bear in a particular situation. If an identity 

is pervasive, it is more likely to be brought 

to bear in numerous situations. The 

researchers hypothesized that individuals 

with more salient and pervasive terrorist 

identities would be more likely to engage in 

terrorism. 

To test this, their analysis drew on data 

from 217 federal cases (involving 465 

unique perpetrators) included in the ATS 

database, which consists of data derived 

primarily from the U.S. federal court 

records of persons indicted for terrorism 

Table 3: Hypothesized Radicalization Pathways (Jensen, 2015)

Pathway Description

Number of 

Individuals Who 

Took Pathway 

(Out of 15)

Trauma Traumatic experience often produces a need to develop meaningful personal relationships, 

which are sometimes found in people with extreme views. Grievances play an important role 

in the development of violent narratives but are not the original impetus for radicalization.

3

IdentitySeekingIdentity-seeking often begins in adolescence and leads to risk-taking behaviors, especially 

juvenile crime. Extremist narratives and groups may offer a life-changing course, sense of 

community, and purpose.

6

Material 

Reward

Economic distress or crisis leads to the desire to increase one’s status or material well-being, 

and extremist groups may offer a means for doing so. 0

Recruitment Extremist narratives/groups are introduced through close personal relationships. Group 

dynamics, especially ingroup/outgroup bias, may play an important role in the development of 

support for violent narratives.

4

15 Although the figure focuses on the process of identity construction among individuals who are members of 

formal groups, the researchers argue that a similar process would occur among those who are not.
How Radicalization to Terrorism Occurs in the United States 11

National Institute of Justice | NIJ.ojp.gov

or terrorism-related activities since 

1980. They used the number of meetings 

attended and how long individuals had 

been group members as proxy measures 

for identity salience, and they used rank in 

the group as a proxy measure for identity 

pervasiveness.16 In support of their 

hypothesis, they found that individuals who 

attended more group meetings and had 

been group members for longer periods 

of time committed a larger number of 

terrorist incidents and were charged with 

more criminal counts. In partial support 

of their hypothesis, they found that 

individuals with higher ranks were charged 

with more criminal counts; they did not, 

however, commit a larger number of 

terrorist incidents (Smith et al., 2016). 

The third concept in Figure 2 that is 

important to understand is identity work, 

which consists of activities that create, 

present, and sustain specific identities 

(Snow & Anderson, 1987). In general terms, 

identity work may involve how a person 

looks, what he or she says and does, and 

with whom he or she associates. 

One specific component of identity work 

involves engaging in demonstration events, 

which are public displays that confirm 

individuals’ commitments to their 

identities. The researchers argued that, 

in the context of terrorist groups, the 

best examples of demonstration events 

include engaging in preparatory activities 

associated with terrorist incidents, and they 

hypothesized that the more demonstration 

events in which individuals participated, 

the more likely they would be to engage 

in terrorism. Again, analysis drew on data 

from 217 federal cases in the ATS database. 

In partial support of their hypothesis, 

the researchers found that individuals 

who participated in a larger number of 

preparatory activities had a larger number 

of criminal counts charged against them. 

They did not, however, commit a larger 

number of terrorist incidents (Smith et 

al., 2016).

Another specific component of identity 

work is identity talk, which involves claiming 

an identity, as well as framing the issues or 

situations that a group or movement seeks 

Figure 2: Identity Convergence (Smith et al., 2016, p. 3)

Identity Appropriation

“Bloc Recruitment”

Identity Work

(Demonstration Events)

Framing Processes

(Diagnostic, Prognostic, Motivational)

Group

Membership Identity Construction

Identity Seekers

“Individual Recruitment”

Salience

Pervasiveness

16 If the team was unable to find data related to whether an individual exhibited a particular characteristic or 

behavior, the individual was removed from any analysis that examined that characteristic or behavior.
How Radicalization to Terrorism Occurs in the United States 12

National Institute of Justice | NIJ.ojp.gov

to address. Diagnostic framing focuses on 

who is viewed as responsible or to blame 

for a situation, prognostic framing focuses 

on what needs to be done to address it, 

and motivational framing focuses on why 

it is important to address (Benford & 

Snow, 2000). There may be competing 

frames within a group (e.g., members may 

view different people as responsible for 

a situation) or, conversely, everyone may 

agree on the same frames. The latter occurs 

through the process of frame crystallization. 

The researchers hypothesized that frame 

crystallization would be a necessary, 

although not sufficient, condition for 

extremist action, including violence. 

To test this hypothesis, the team focused 

on analyzing terrorist groups as opposed 

to terrorist individuals.17 Specifically, 

the researchers conducted qualitative 

analysis using data from 43 federally 

indicted organizations included in 

the ATS database, supplementing this 

material with additional data based on 

media reports, police records, and other 

open-source information (Tan et al., 

2015). They coded the levels of these 

organizations’ frame crystallization, as well 

as other organizational and contextual 

characteristics, such as organization size 

and duration, political context, and type 

of recruitment (open or closed). They 

then analyzed these data using qualitative 

comparative and fuzzy-set analysis (Ragin, 

2008), which allows for the identification 

of multiple pathways to the same outcome. 

In support of their hypotheses, they found 

that frame crystallization most consistently 

contributed to violence against people and 

property across a range of organizations. 

On the other hand, while characteristics 

like organization size and duration, 

political context, and recruitment style also 

contributed to violent outcomes, they did 

not do so as consistently across different 

organizations or types of violence (against 

people versus against property) (Smith 

et al., 2016). In other words, there were 

some differences in the pathways taken 

to violence based on the type of violence 

perpetrated and the organization.

In summary, the findings from this research 

suggest the importance of the identity 

construction process in radicalization to 

terrorism and the need to understand 

how groups frame their beliefs. They also 

suggest that the conditions that facilitate 

radicalization to terrorism may vary by 

organization and type of violent outcome.

General Insights Into the 

Process of Radicalization 

to Terrorism and Their 

Implications

On the surface, what initially may be 

most striking about the four NIJ-funded 

projects discussed above are the different 

approaches they take to examining the 

process of radicalization to terrorism. 

Specifically, they range from efforts that 

test the relevance of existing theories 

and frameworks of radicalization to 

efforts that identify new models and 

pathways; from research focused on 

particular types of terrorists to research 

that includes individuals characterized 

by varied extremist ideologies, tactics, 

and relationships to formal groups; from 

efforts that examine specific aspects of 

the radicalization process to efforts that 

attempt to examine the process as a whole; 

and from research that focuses on the 

sequence of activities individuals engage in 

as they radicalize to research that focuses 

on the prevalence of certain radicalization 

mechanisms and activities.

17 It is important to note that it cannot be assumed that all individuals in a particular group embrace the beliefs 

that are held by the group as a whole, although higher levels of frame crystallization could indicate that this is 

more likely to be the case.
How Radicalization to Terrorism Occurs in the United States 13

National Institute of Justice | NIJ.ojp.gov

Given this variation, it should not be 

surprising that the more specific findings 

of the projects — e.g., that any preradicalization indicator followed by offline 

peer immersion and then expression of 

the desire for action was often observed 

among U.S. homegrown offenders inspired 

by or affiliated with al-Qa’ida, and that 

lone wolves tended to combine personal 

and political grievances — are likely to be 

most relevant to practitioners focused on 

the same particular types of terrorists and/

or aspects of the radicalization process that 

the researchers examined.

What may be more surprising given the 

variation in the projects is the significant 

overlap in their findings regarding the 

general facilitators of radicalization to 

terrorism and the importance of examining 

how the process evolves within individuals, 

potentially manifesting in changes in 

behaviors over time. The projects also 

underline the need to move beyond onesize-fits-all approaches to understanding 

this process. These collective findings 

may be particularly important given 

that they were supported by diverse and 

rigorous quantitative and qualitative data 

collection efforts and the analysis of data 

that covered different subsets of terrorists. 

These findings, along with some of their 

possible implications for prevention and 

intervention efforts, are described below.

Facilitators of Radicalization to 

Terrorism

The findings from the four NIJ-sponsored 

research teams highlight the importance 

of several facilitators that may encourage, 

support, or contribute to the process 

of radicalization to terrorism. These 

facilitators are outlined in Table 4 and 

described below.

The potential role of terrorist belief 

systems and narratives in the process of 

radicalization to terrorism was strongly 

supported by research sponsored by NIJ 

(Klausen, 2015b; Sawyer, 2016; Smith et 

al., 2016). For example, in one analysis 

using the PIRUS database, the START 

team examined the relationship between 

measures of the strength of individuals’ 

extremist beliefs and a variety of different 

extremist behaviors and found that 74 

percent of the 675 individuals who trained 

or engaged in terrorism were deeply 

committed to a terrorist ideology (Sawyer, 

2016). In addition, by demonstrating that 

frame crystallization (i.e., identifying and 

agreeing on who is to blame for a situation 

and what needs to be done to address it) 

is a facilitator of terrorism, the team led 

by the University of Arkansas underlined 

the important role that terrorist group 

narratives may play in the radicalization 

process (Smith et al., 2016). At the same 

time, while the findings from these projects 

Table 4: Facilitators of Radicalization Supported by Multiple NIJ-Sponsored 

Projects

Facilitators 

Terrorist belief systems or narratives

Identity processes

Activities that demonstrate commitment to a terrorist group or cause

Connections with terrorists in one’s offline social network

Connections with terrorists via the internet and/or social media

Group dynamics

Grievances

Triggering events
How Radicalization to Terrorism Occurs in the United States 14

National Institute of Justice | NIJ.ojp.gov

indicate that individuals’ beliefs and 

narratives are important to understand, 

they are consistent with other research that 

has demonstrated that embracing terrorist 

beliefs or narratives is neither a necessary 

nor sufficient condition for radicalization 

to terrorism (e.g., Horgan, 2014; Lloyd 

& Dean, 2015). As a consequence, 

practitioners may also need to address other 

facilitators of this process in prevention and 

intervention efforts. 

The role of identity processes in 

radicalization to terrorism has been 

highlighted in previous research (e.g., 

Simi, Bubolz, & Hardman, 2013; Weine 

& Ahmed, 2012) and was empirically 

supported in the projects sponsored 

by NIJ (Jensen, 2015; Smith et al., 

2016). Experiencing identity conflict 

or confusion — whether because of a 

struggle to adapt to a new culture, one’s 

stage of life (e.g., adolescence), or other 

challenges — is often viewed as potentially 

leaving individuals more open to adopting 

new ideas and behaviors, including those 

associated with terrorism. The team led 

by the University of Arkansas (Smith et al., 

2016) went one step further by examining 

how individuals construct their terrorist 

identities and found some evidence that 

when these identities are both salient to 

individuals and pervade numerous aspects 

of their lives, they were more likely to 

engage in terrorism. One implication 

of this is that programs developed to 

prevent or intervene in the radicalization 

process may need to consider facilitating 

individuals’ recognition or development of 

alternate identities, such as their identities 

as parents, children, or citizens.

Activities that demonstrate commitment 

to a terrorist group or cause were found 

to facilitate radicalization to terrorism 

in two NIJ-sponsored research projects 

(Hamm & Spaaij, 2015; Smith et al., 

2016). The team led by the University 

of Arkansas found that individuals 

who participated in a larger number of 

demonstration events (i.e., public displays 

that confirm individuals’ commitments 

to their identities) had more criminal 

counts charged against them (Smith et al., 

2016), while the team from Indiana State 

University and Victoria University found 

that lone wolves tended to broadcast their 

intent before engaging in violence (Hamm 

& Spaaij, 2015). A key implication of these 

findings is that a better understanding 

of these activities, which often occur in 

public, may provide community members 

and practitioners with additional 

opportunities to identify individuals 

who are radicalizing to terrorism and to 

intervene before an attack occurs.

Having connections with terrorists in 

one’s offline social network has also been 

posited as a facilitator of radicalization to 

terrorism in past research (e.g., McCauley 

& Moskalenko, 2011; Sageman, 2004), and 

NIJ-sponsored research provided strong 

evidence that having friends and family 

members who embrace terrorism can 

influence an individual’s choice to become 

(or remain) a terrorist (Hamm & Spaaij, 

2015; Klausen, 2015b; LaFree, 2015; Sawyer, 

2016). For example, in the PIRUS data, 

the START team found that there was an 

extremely strong correlation between the 

radicalization outcomes of individuals and 

those in their close social networks, with a 

third of individuals engaging in the same 

extremist behaviors as someone else in their 

network (Sawyer, 2016). Further, although 

familial and romantic relationships were 

related to these outcomes, friends were 

much more likely to end up engaging in 

the same kinds of extremist behaviors. One 

implication of this is that when trying to 

identify individuals who may radicalize to 

terrorism or when designing intervention 

programs, it may be important to pay 

attention to individuals’ offline social 

networks, and particularly their friends.

Similarly, a growing body of research 

has highlighted the role that having 

connections with terrorists via the 
How Radicalization to Terrorism Occurs in the United States 15

National Institute of Justice | NIJ.ojp.gov

internet and social media may play in 

radicalization to terrorism (e.g., Neumann, 

2013; Von Behr et al., 2013). In addition 

to potentially leading individuals to 

become more accepting of violence and 

fostering feelings of closeness with those 

who perpetrate it, virtual connections may 

provide practical guidance that facilitates 

terrorism. NIJ-sponsored projects provided 

empirical evidence of the importance of 

this facilitator (Hamm & Spaaij, 2015; 

LaFree, 2015; Klausen, 2015b; Klausen 

et al., 2013). For example, the Brandeis 

University team discussed the changes that 

social media have wrought among those 

inspired by or affiliated with al-Qa’ida 

in the U.S. and demonstrated that shifts 

in where recruitment occurs — away 

from geographic locations and toward 

social media sites — have led to U.S. 

recruits being better integrated with their 

international counterparts (Klausen et al., 

2013).18 On the one hand, this may make 

it more difficult for community members 

and practitioners to identify individuals 

who are radicalizing to terrorism, as they 

may no longer be co-located in geographic 

clusters. In this sense, the development 

of robust international partnerships 

among practitioners becomes even more 

important. On the other hand, assuming 

that at least some of these connections 

occur on open social media, they may 

serve as an additional means of identifying 

individuals who may be radicalizing to 

terrorism.

The findings from NIJ-sponsored research 

also provided empirical evidence that 

group dynamics may serve as a facilitator 

of the process of radicalization to terrorism 

(Jensen, 2015; Klausen, 2015b). This 

supports previous research that has argued 

that belonging to a tight-knit group may 

lead to individuals being more likely to 

(1) accept their fellow group members’ 

views and (2) consider those inside of 

their group more positively (ingroup 

favoritism) and those outside of their group 

more negatively (outgroup derogation) 

(McCauley & Moskalenko, 2011; Sageman, 

2004; Smith, 2008). The important role that 

group dynamics may play in the process 

of radicalization to terrorism is perhaps 

clearest in the Brandeis University team’s 

research, which documented the role of 

peer immersion (i.e., becoming closer to 

a peer group of like-minded individuals) 

in the radicalization process (Klausen, 

2015b). Importantly, the team also found 

evidence that individuals detach from 

their previous lives as they radicalize, 

which has implications for identifying 

those who may be at risk. Specifically, 

it highlights that those who know an 

individual’s typical behaviors (e.g., those 

close to him or her) may be best positioned 

to identify changes in behaviors that 

are possibly linked to radicalization to 

terrorism. The role of group dynamics in 

radicalization also implies that prevention 

and intervention programs might benefit 

by adapting existing approaches aimed at 

reducing other forms of ingroup bias and 

outgroup derogation (e.g., prejudice and 

discrimination).

Numerous researchers have argued that 

grievances play a role in radicalization to 

terrorism (e.g., Borum, 2003; McCauley 

& Moskalenko, 2011; Moghaddam, 2005), 

and this view was empirically supported in 

NIJ-sponsored research (Hamm & Spaaij, 

2015; LaFree, 2015; Smith et al., 2016). 

Feeling that one (or one’s group) has been 

treated unfairly, discriminated against, or 

targeted by others may lead individuals to 

seek justice or revenge against those they 

blame for this situation. For example, the 

researchers from Indiana State University 

and Victoria University (Hamm & Spaaij, 

2015) discussed the particular nature that 

18 It should be noted that the finding that recruitment occurs less frequently in specific geographic locations 

may be somewhat unique to the U.S., as there is evidence that the same shift may not be occurring in Europe 

(Neumann, 2015). 
How Radicalization to Terrorism Occurs in the United States 16

National Institute of Justice | NIJ.ojp.gov

these grievances may take among lone 

wolves. Specifically, they argued that 

lone wolves are often socially excluded, 

leading them to (1) feel that they are 

being deprived of what is their due and 

(2) form grievances against those they 

view as responsible for this injustice. Along 

the same lines, the researchers noted that 

lone wolves often combine personal and 

political grievances, with one implication 

being that the grievances of lone wolves 

may sound different — and perhaps more 

idiosyncratic — than those of group actors.

Finally, the role of triggering events (i.e., 

crises or events that may lead individuals 

to feel pressured to act) in the process of 

radicalization to terrorism was empirically 

supported by NIJ-sponsored research 

(Hamm & Spaaij, 2015; LaFree, 2015; 

Klausen, 2015b). For example, in their 

research on lone wolves, the team from 

Indiana State University and Victoria 

University found that triggering events 

occurred in the vast majority of the cases 

they examined and that these events 

ranged from personal to political to some 

combination of the two (Hamm & Spaaij, 

2015). They also noted that, in some cases, 

triggering events were immediate catalysts 

for action, and in others they slowly 

accumulated over time. While it may be 

difficult to identify which specific events 

may contribute to an individual engaging 

in terrorism, these findings highlight the 

need for practitioners to take into account 

aspects of individuals’ larger personal, 

political, and social contexts in their 

prevention and intervention efforts.

The Diversity of Radicalization 

Processes and Pathways

As discussed above, the research sponsored 

by NIJ found empirical support for several 

facilitators of the process of radicalization 

to terrorism. In addition, both of the 

projects that focused on specific subsets 

of terrorists identified commonalities 

in how these individuals radicalized, as 

well as in their observable behaviors. 

The Brandeis University team found that 

U.S. homegrown offenders inspired by 

or affiliated with al-Qa’ida tended to 

engage in specific sequences of behaviors 

associated with radicalization to terrorism 

(Klausen et al., 2015; Klausen, 2015b), and 

the team from Indiana State University and 

Victoria University observed specific, albeit 

somewhat different, patterns of behaviors 

in U.S. lone wolves (Hamm & Spaaij, 2015). 

These findings all provide initial guidance 

for community members and practitioners 

attempting to identify individuals who may 

be radicalizing to terrorism, as well as those 

developing programs focused on preventing 

and intervening in this process. 

At the same time, another important 

common finding — both explicit and 

implicit — across the projects examined 

in this paper is that there are multiple 

pathways to terrorism and that these 

pathways may vary by the extremist 

ideologies and narratives individuals 

embrace, the time periods in which they 

radicalize, the groups or movements they 

join (or don’t join in the case of lone 

wolves), and/or individuals’ characteristics 

and experiences.

For example, the quantitative analysis 

conducted by the START team using the 

PIRUS database identified differences in 

the prevalence of particular radicalization 

mechanisms based on the particular 

extremist belief system (LaFree, 2015). 

Specifically, the team found that, compared 

with those who embraced other belief 

systems, Islamist terrorists were more likely 

to be radicalized through the internet, 

through tight-knit cliques, and/or through 

grievances related to particular events.

Importantly, however, the START team also 

cautioned that some of these findings may 

be impacted by the time periods in which 

these terrorists tended to be active (LaFree, 

2015). The fact that the Islamist terrorists 

included in the database tended to have 
How Radicalization to Terrorism Occurs in the United States 17

National Institute of Justice | NIJ.ojp.gov

radicalized since 2001 may help to explain 

the finding that Islamist terrorists were 

more likely to be radicalized through the 

internet. Similarly, the team from Indiana 

State University and Victoria University 

found differences in the radicalization 

processes of lone wolves in the U.S. based 

on whether they radicalized before or after 

9/11 (Hamm & Spaaij, 2015). Compared 

with those who committed attacks before 

9/11, those who committed them after 

9/11 were less likely to have an affinity 

with an extremist group and more likely 

to be enabled by individuals who assisted 

them in planning attacks and/or who 

provided them with inspiration. Finally, the 

Brandeis team found that individuals who 

radicalized before 2010 typically took five to 

six years to move from initial exploration of 

extremist ideas to a terrorist offense, while 

those who radicalized during or after 2010 

typically took 19 months (Klausen, 2015b). 

These findings all point to the importance 

of continuing to study the process of 

radicalization to terrorism, as it may change 

over time. 

It is also the case that whether an individual 

belongs to a group or is a lone wolf may 

impact the radicalization process. The 

most obvious example relates to the 

types of group dynamics that have been 

demonstrated to facilitate radicalization 

to terrorism within terrorist groups and 

movements (e.g., Jensen, 2015; Klausen, 

2015b) but likely have a lesser impact on 

lone wolves. In addition, there may be 

differences in some of the commitmentdemonstrating activities in which lone 

wolves and extremist group members 

participate (e.g., lone wolves would 

arguably be less likely to demonstrate 

their commitment to a cause by 

participating in group meetings or 

initiation rituals). Finally, as the team 

from Indiana State University and 

Victoria University pointed out, one of 

the signatures of lone wolves is that they 

combine personal and political grievances 

(Hamm & Spaaij, 2015), and it is not clear 

whether this combination is as prevalent 

among members of terrorist groups. 

Again, however, it is important to keep in 

mind that just because the radicalization 

processes of lone actors and extremist 

group members may differ, this does not 

mean that the radicalization processes of all 

group members are the same. As discussed 

above, there may be variations based on 

the extremist belief system (LaFree, 2015). 

Further, based on the research conducted 

by the University of Arkansas-led team 

(Smith et al., 2016), there may also be 

differences in the radicalization processes 

of different organizations that share similar 

belief systems. 

Finally, while the findings above did 

demonstrate the possibility of developing 

frameworks and models that explain how 

specific subsets of individuals radicalize to 

terrorism (e.g., U.S. homegrown offenders 

inspired by or affiliated with al-Qa’ida, 

lone wolves), it should not be expected that 

every individual who meets these specific 

criteria follows the exact same pathway. 

This is clear in the statistics presented by 

the Brandeis team (Klausen, 2015b) and 

the team from Indiana State University 

and Victoria University (Hamm & Spaaij, 

2015). To give a concrete example, the 

fact that 80 percent of lone wolves in the 

U.S. combined personal and political 

grievances is impressive and indicates 

that the vast majority of lone wolves did 

so. It also indicates that a small minority 

did not. This is not intended to diminish 

this finding or any of the other findings 

discussed above — they are evidencebased and can provide guidance to those 

working to identify that radicalization to 

terrorism is occurring and/or to prevent or 

intervene in this process. Rather, it serves as 

a reminder that the radicalization process 

is complex and varied and, thus, will never 

be amenable to one-size-fits-all solutions. 

The implication is that while community 
How Radicalization to Terrorism Occurs in the United States 18

National Institute of Justice | NIJ.ojp.gov

members and practitioners can and should 

be guided by research and experience, they 

will still need to consider the characteristics 

and experiences that may set a particular 

individual apart. 

Overview of Findings and 

Next Steps

The projects sponsored by NIJ have taken 

a variety of approaches to examining the 

process of radicalization to terrorism. 

These include testing whether an existing 

framework of radicalization was supported 

by new data on U.S. homegrown offenders 

inspired by or affiliated with al-Qa’ida, 

developing a new model of radicalization 

to terrorism among lone wolves in the U.S., 

using quantitative and qualitative analysis 

to examine the radicalization processes 

and trajectories of U.S. terrorists motivated 

by a range of ideologies, and testing 

whether two theoretical perspectives on 

radicalization to terrorism were supported 

by data on individuals and groups indicted 

for terrorism or terrorism-related activities 

in U.S. federal courts.

In spite of these different approaches, 

there is substantial overlap in the findings 

from these projects, which collectively 

provide evidence of the importance of 

several facilitators of radicalization to 

terrorism that occur within individuals, in 

interaction with other individuals, and at 

the broader community or societal level. 

At the individual level, the radicalization 

process often involves embracing a terrorist 

belief system or narrative that identifies 

particular others or groups as “enemies” 

and justifies engaging in violence against 

them. Individuals may also begin to 

identify themselves as terrorists as well 

as engage in activities that highlight 

their commitments to their new beliefs, 

identities, and/or others who hold them. 

It is, however, important to note that 

while these beliefs and behaviors may 

facilitate the movement to terrorism, this 

outcome is not inevitable. Those close to 

these individuals may become aware of 

the changes that their friends and family 

members are undergoing and attempt to 

address them or seek help from others who 

can. An important implication of this is that 

trusted information and resources need to 

be available to assist in this effort. Another 

is that prevention and intervention efforts 

may benefit by addressing beliefs that justify 

violence and helping individuals to develop 

identities in which these beliefs are not 

central. 

The research sponsored by NIJ 

also highlights the social nature of 

radicalization to terrorism and the roles 

that connections with terrorists (online 

and/or offline) and group dynamics 

may play in this process. As individuals’ 

relationships with others who support 

terrorism become stronger, they may begin 

separating themselves from those who do 

not, thus becoming increasingly isolated 

from people who might challenge their 

views. This suggests that dramatic changes 

in the people with whom an individual 

associates, or increasing insularity among 

existing groups of friends, may be causes for 

concern and that it may be those close (or 

previously close) to these individuals who 

are best positioned to notice these changes. 

It also suggests that efforts to prevent or 

intervene in the radicalization process 

must take into account both the individuals 

and those with whom they interact, as 

well as potentially facilitate establishing 

or re-establishing their relationships with 

nonextremists.

Finally, there is evidence that events 

in the larger community and societal 

contexts may also facilitate radicalization 

to terrorism. For example, individuals 

may experience real or perceived 

grievances — both personal and political — 

that may help to fuel their movement toward 

terrorism. Further, in some cases, specific 

triggering events associated with these 

grievances may accelerate this movement. 
How Radicalization to Terrorism Occurs in the United States 19

National Institute of Justice | NIJ.ojp.gov

Thus, understanding these grievances 

and triggering events, as well as helping 

individuals to develop constructive ways to 

address them, may be important components 

of programs developed to prevent and 

intervene in the radicalization process. 

It is, however, important to keep in mind 

that in addition to identifying several 

common facilitators of radicalization to 

terrorism and underlining the need to 

take into account how this process unfolds 

within individuals over time, the research 

sponsored by NIJ also provides empirical 

evidence that individuals’ processes of 

radicalization to terrorism may vary by 

the extremist ideologies and narratives 

they embrace, the time periods in which 

they radicalize, the groups or movements 

they join (or do not join, in the case of 

lone wolves), and/or their individual 

characteristics and experiences. Thus, while 

community members and practitioners can 

benefit from the evidence-based guidance 

provided by this research, it will continue 

to remain important that they take into 

consideration the specific characteristics 

and experiences of the individuals with 

whom they are working.

It will also remain important to conduct 

additional research into the similarities 

and differences in the processes of 

radicalization to terrorism among 

individuals who vary in all of the ways 

discussed above. The NIJ-sponsored 

projects discussed in this paper have 

highlighted some of these similarities and 

differences, but there is still much to learn. 

For example, do additional facilitators 

of radicalization to terrorism — and 

sequences of facilitators — emerge from 

in-depth studies of extremists inspired 

by or affiliated with anti-government, 

anti-capitalist, nativist, or other political 

and social terrorist movements? Do 

individuals who radicalize outside of the 

U.S. experience different facilitators than 

those who radicalize inside the U.S.? Will 

the facilitators of radicalization to terrorism 

continue to evolve among lone-wolf 

terrorists (and others) as technologies and 

time advance, and, if so, how? Are certain 

facilitators of the radicalization process 

associated with a greater likelihood that 

individuals will go on to commit terrorist 

offenses? How do individual characteristics 

and experiences interact with facilitators 

of the radicalization processes? For 

example, are individuals with certain 

characteristics or experiences more or less 

likely to be exposed to or influenced by 

specific facilitators of the radicalization 

process? And, perhaps most importantly, 

what are the facilitators of moving away 

from terrorism, and how can they be best 

supported?

Finally, it will be necessary to conduct more 

research that compares individuals who 

radicalize to terrorism with those who do 

not. In addition to allowing for a deeper 

understanding of how the radicalization 

process unfolds, this research will provide 

essential information on its ultimate causes 

and how prevention and intervention efforts 

can best address them.
How Radicalization to Terrorism Occurs in the United States 20

National Institute of Justice | NIJ.ojp.gov

References

Benford, R., & Snow, D. (2000). Framing processes and social movements: An overview and 

assessment. Annual Review of Sociology, 26, 611-639.

Berger, J. (2011). Jihad Joe: Americans who go to war in the name of Islam. Washington, D.C.: 

Potomac Books, Inc.

Borum, R. (2003). Understanding the terrorist mindset. FBI Law Enforcement Bulletin, 72, 7-10.

Canadian Network for Research on Terrorism, Security, and Society (2016). Canadian 

Incident Database. 

Feve, S., & Bjornsgaard, K. (2016). Lone-actor terrorism: Database workshop. London: Royal 

United Services Institute.

Freilich, J., et al. (2014). Introducing the United States Extremist Crime Database (ECDB). 

Terrorism and Political Violence, 26, 372-384.

Gill, P. (2015). Lone-actor terrorists: A behavioural analysis. London: Routledge.

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

radicalization pathways to forge prevention strategies. Final grant report to NIJ. Available at: 

www.ncjrs.gov/pdffiles1/nij/grants/248691.pdf.

Hogg, M.A., & Adelman, J. (2013). Uncertainty-identity theory: Extreme groups, radical 

behaviour, and authoritarian leadership. Journal of Social Issues, 69, 436-454. 

Horgan, J. (2014). The psychology of terrorism, 2nd edition. Abingdon, England: Routledge.

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. 

Final grant report to NIJ. Available at: www.ncjrs.gov/pdffiles1/nij/grants/249937.pdf.
How Radicalization to Terrorism Occurs in the United States 21

National Institute of Justice | NIJ.ojp.gov

Jensen, M. (2015). The empirical assessment of domestic radicalization: A qualitative 

approach. In J. Picarelli (Chair), Violent extremism in the U.S. and lessons about combating 

and preventing it. Session conducted at the annual meeting of the American Society of 

Criminology, Washington, D.C., November 2015.

Jensen, M. (2016). Personal communication with the author.

Klausen, J. (2012). Al Qaeda’s London branch: Patterns of domestic and transnational 

network integration. Studies in Conflict & Terrorism, 35, 411-431.

Klausen, J. (2015a). Personal communication with the author.

Klausen, J. (2015b). Tweeting the Jihad: Social media networks of western foreign fighters in 

Syria and Iraq. Studies in Conflict & Terrorism, 38, 1-22.

Klausen, J., et al. (2013). Mapping Islamist terror networks in the United States: Hamas, 

Hezbollah, and Jihadist groups — 1993-2013. Unpublished manuscript, Brandeis University.

Klausen, J., et al. (2015). Toward a behavioral model of “homegrown” radicalization 

trajectories. Studies in Conflict & Terrorism, 39, 67-83.

Kruglanski, A.W., et al. (2009). Fully committed: Suicide bombers’ motivation and the quest 

for personal significance. Political Psychology, 30, 331-357.

LaFree, G. (2015). 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.

LaFree, G., & Dugan, L. (2007). Introducing the Global Terrorism Database. Terrorism and 

Political Violence, 19, 181-204.

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.

McCauley, C., & Moskalenko, S. (2011). Friction: How radicalization happens to them and us.

New York: Oxford University Press. 

Moghaddam, F. (2005). The staircase to terrorism: A psychological exploration. American 

Psychologist, 60, 161-169.

Nesser, P. (2014). Islamist terrorism in Europe. London: C. Hurst & Co. Publishers. 

Neumann, P. (2013). Options and strategies for countering online radicalization in the 

United States. Studies in Conflict & Terrorism, 36, 431-459. 

Neumann, P. (2015). ISIS’s social media campaign: Myth and reality. In S. Hughes (Chair), 

Can we identify radicalization as it happens? Plenary session conducted at the conference 

Radicalization and Violent Extremism: Lessons Learned from Canada, the United 

Kingdom, and the US, Arlington, Va., July 2015.
How Radicalization to Terrorism Occurs in the United States 22

National Institute of Justice | NIJ.ojp.gov

Ragin, C. (2008). Rethinking social inquiry: Fuzzy sets and beyond. Chicago: University of 

Chicago Press.

Sageman, M. (2004). 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.

Silber, M., & Bhatt, A. (2007). Radicalization in the West: The homegrown threat. New York: 

New York City Police Department.

Simi, P., Bubolz, B., & Hardman, A. (2013). Military experience, identity discrepancies, and 

far right terrorism: An exploratory analysis. Studies in Conflict & Terrorism, 36, 654-671.

Smith, A. (2008). The implicit motives of terrorist groups: How the needs for affiliation and 

power translate into death and destruction. Political Psychology, 29, 55-75.

Smith, B., & Damphousse, K. (2009). Patterns of precursor behaviors in the lifespan of an 

American eco-terrorist group. Criminology & Public Policy, 8, 475-496.

Smith, B., Damphousse, K., & Roberts, P. (2006). Pre-incident indicators of terrorist incidents: 

The identification of behavioral, geographic, and temporal patterns of preparatory conduct. Final 

grant report to NIJ. Available at: https://www.ncjrs.gov/pdffiles1/nij/grants/214217.pdf. 

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

process. Final grant report to NIJ. Available at: https://www.ncjrs.gov/pdffiles1/nij/

grants/249673.pdf. 

Smith, B., Roberts, P., & Damphousse, K. (2013). Update on geospatial patterns of antecedent 

behavior among perpetrators in the American Terrorism Study (ATS). Report to the Resilient 

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

Security. College Park, MD: START. 

Snow, D.A., & Anderson, L. (1987). Identity work among the homeless: The verbal 

construction and avowal of personal identities. American Journal of Sociology, 92, 1336-1371.

Sullivan, B., Freilich, J., & Chermak, S. (2014). Financial crime and material support schemes 

linked to al-Qa’ida and affiliated movements (AQAM) in the United States: 1990 to June 

2014. Report to the Resilient Systems Division, Science and Technology Directorate, U.S. 

Department of Homeland Security. College Park, MD: START. 

Tan, A., et al. (2015). Facilitative conditions for “domestic terrorism”: Extremist political violence 

in the United States, 1980-2012. Unpublished presentation.

Taylor, M., & Quayle, E. (1994). Terrorist lives. London: Brassey’s.
How Radicalization to Terrorism Occurs in the United States 23

National Institute of Justice | NIJ.ojp.gov

Von Behr, I., et al. (2013). Radicalisation in the digital era: The use of the internet in 15 cases of 

terrorism and extremism. Washington, D.C.: Rand. 

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

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

College Park, MD: START.