Since the terrorist attacks of September 11, 2001 (9/11), domestic terrorists— people who commit crimes within the homeland and draw inspiration from U.S.-based extremist ideologies and movements —have not received as much attention from federal law enforcement as their violent jihadist counterparts. This was not necessarily always the case. The Federal Bureau of Investigation (FBI) reported in 1999 that "[d]uring the past 30 years, the vast majority—but not all—of the deadly terrorist attacks occurring in the United States have been perpetrated by domestic extremists." The U.S. government reacted to 9/11 by greatly enhancing its counterterrorism efforts. This report discusses how domestic terrorists broadly fit into the counterterrorism landscape, a terrain that since 9/11 has been largely shaped in response to terrorists inspired by foreign ideologies. This report focuses especially on how domestic terrorism is conceptualized by the federal government and issues involved in assessing this threat's significance. Today (perhaps in part because of the government's focus on international terrorist ideologies), it is difficult to evaluate the scope of domestic terrorist activity. For example, federal agencies employ varying terminology and definitions to describe it. Possibly contributing to domestic terrorism's secondary status as a threat at the federal level, a large number of those labeled as domestic terrorists do not necessarily use traditional terrorist tactics such as bombings or airplane hijackings. Additionally, many domestic terrorists do not intend to physically harm people but rather rely on alternative tactics such as theft, trespassing, destruction of property, and burdening U.S. courts with retaliatory legal filings. While plots and attacks by foreign-inspired homegrown violent jihadists have earned more media attention, domestic terrorists have been busy as well. It is worth noting that in terms of casualties on U.S. soil, an act of domestic terrorism is second only to the events of 9/11. Timothy McVeigh's bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995, claimed 168 lives and injured more than 500 others. Some estimates suggest that domestic terrorists are responsible for carrying out dozens of incidents since 9/11. Much like their jihadist counterparts, domestic terrorists are often Internet savvy and use the medium as a resource for their operations. Terrorists are typically driven by particular ideologies. In this respect, domestic terrorists are a widely divergent lot, drawing from a broad array of philosophies and worldviews. These individuals can be motivated to commit crimes in the name of ideas such as animal rights, white supremacy, and abortion, for example. However, the expression of these worldviews (minus the commission of crimes) involves constitutionally protected activity. Aware of the lines between constitutionally protected speech and criminality, domestic terrorists often rope themselves off from ideological (above-ground) elements that openly and often legally espouse similar beliefs. In essence, the practitioners who commit violent acts are distinct from the propagandists who theorize and craft worldviews that could be interpreted to support these acts. Thus, in decentralized fashion, terrorist lone actors (lone wolves) or isolated small groups (cells) generally operate autonomously and in secret, all the while drawing ideological sustenance— not direction —from propagandists operating in the free market of ideas. Domestic terrorists may not be the top federal counterterrorism priority, but they feature prominently among the concerns of some law enforcement officers. For example, in 2011, Los Angeles Deputy Police Chief Michael P. Downing included "black separatists, white supremacist/sovereign citizen extremists, and animal rights terrorists" among his chief counterterrorism concerns. A 2014 national survey of state and local law enforcement officers found that sovereign citizens were "the top concern" among terrorist threats. The violence related to protests in Charlottesville, VA, on August 12, 2017, also has raised the issue of domestic terrorism, particularly related to public discussions regarding a widely reported incident involving James Alex Fields, who according to witnesses drove his car into a group of people protesting a rally featuring white supremacists in Charlottesville on August 12. Fields allegedly killed one person and injured 19 others in the incident. The Department of Justice (DOJ) has opened a civil rights investigation into the incident, presumably pursuing possible hate crime charges. Additionally, Attorney General Jeff Sessions has publicly stated that terrorism investigators are involved in investigating the incident, ostensibly exploring the possibility of characterizing it as an act of domestic terrorism rather than a hate crime. This report provides background regarding domestic terrorists—detailing what constitutes the domestic terrorism threat as suggested by publicly available U.S. government sources. It illustrates some of the key factors involved in assessing this threat. This report does not discuss in detail either violent jihadist-inspired terrorism or the federal government's role in counterterrorism investigations. Two basic questions are key to understanding domestic terrorism. First, what exactly constitutes "domestic terrorism?" Answering this question is more complicated than it may appear. Some consider all terrorist plots occurring within the homeland as acts of domestic terrorism. According to this perspective, a bombing plot involving U.S. citizens motivated by a foreign terrorist group such as Al Qaeda or the Islamic State constitutes domestic terrorism. While this conceptualization may be true at some level, a practical definition of domestic terrorism distilled from federal sources is much narrower. It suggests that domestic terrorists are Americans who commit ideologically driven crimes in the United States but lack foreign direction or influence—whether tactical or philosophical. This conceptualization excludes homegrown individuals directed or motivated by groups such as Al Qaeda or the Islamic State. Second, what particular groups are considered domestic terrorist organizations? The U.S. government does not provide a precise, comprehensive, and public answer to this question. Rather, in broad terms, DOJ has identified a number of general threats that embody this issue. The ideological concepts that underpin such threats may inspire criminal activity, such as hate crimes, that do not rise to the level of terrorism. This further complicates defining "domestic terrorism." In the most general statutory terms, a domestic terrorist engages in terrorist activity that occurs in the homeland. The Federal Bureau of Investigation (FBI, the Bureau) has lead responsibility for terrorism investigations at the federal level. The FBI generally relies on two fundamental sources to define domestic terrorism. First, the Code of Federal Regulations characterizes "terrorism" as including "the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives." Second, 18 U.S.C. Section 2331(5) more narrowly defines "domestic terrorism" and differentiates it from international terrorism and other criminal activity. This definition comes from Section 802 of the USA PATRIOT Act ( P.L. 107-56 ). According to 18 U.S.C. Section 2331(5), domestic terrorism occurs primarily within U.S. territorial jurisdiction, and involves (A) ... acts dangerous to human life that are a violation of the criminal laws of the United States or of any State; (B) appear to be intended— (i) to intimidate or coerce a civilian population; (ii) to influence the policy of a government by intimidation or coercion; or (iii) to affect the conduct of a government by mass destruction, assassination, or kidnapping.... The definitions cited above are too broad to capture what the FBI specifically investigates as "domestic terrorism." Besides the statutory definitions regarding the crime of domestic terrorism, the FBI has historically emphasized particular qualities inherent to the actors who engage in domestic terrorism. According to the Bureau, domestic terrorists do not simply operate in the homeland, but they also lack foreign direction . In fact, the Bureau's practical, shorthand definition of domestic terrorism is "Americans attacking Americans based on U.S.-based extremist ideologies." The Department of Homeland Security (DHS) follows this construction. On the surface, the FBI's shorthand definition for domestic terrorism appears straightforward. However, there is inherent ambiguity to it. Namely, some of the "U.S.-based extremist ideologies" driving what the Bureau views as domestic terrorism have international roots and active followings abroad. The ideologies supporting eco-extremism and animal rights extremism (discussed below) readily come to mind, and people have long committed crimes in their names outside the United States. At least in part, their origins lay in the United Kingdom. Nazism—with its German origins and foreign believers—is an element within domestic white supremacist extremism. Anarchism, the philosophy followed by anarchist extremists, also has long-standing European roots. The racist skinhead movement traces its origins abroad—to the United Kingdom—as well. These examples illustrate the FBI's challenge when it emphasizes U.S.-based ideologies in its framing of domestic terrorism. A few more issues make it hard to grasp the breadth of domestic terrorist activity in the United States. First, counting the number of terrorist prosecutions or plots in general has been difficult in the post-9/11 period. Second, there may be some ambiguity in the investigative process regarding exactly when criminal activity becomes domestic terrorism. Third, the federal government appears to use the terms "terrorist" and "extremist" interchangeably when referring to domestic terrorism. It is unclear why this is the case. Finally, and most importantly, which specific groups are and should be considered domestic terrorist organizations? The U.S. government does not provide a public answer to this question. Rather, the federal government defines the issue in terms of "threats," not groups. While statutory and practical federal definitions exist for "domestic terrorism," there is little clear sense of the scope of the domestic terrorist threat based on publicly available U.S. government information.