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 foreign counterparts inspired by Al Qaeda. This was not necessarily always the case. The 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 this new counterterrorism landscape, a terrain that in the last 10 years 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. Also, domestic terrorism-related intelligence collection efforts have not necessarily received the same attention as similar efforts to counter foreign threats. Beyond these issues, the Obama Administration's community outreach-driven strategy to quell terrorism-related radicalization in the United States focuses on individuals inspired by Al Qaeda. How domestic terrorism fits into this strategy is unclear. Congress may opt to examine these and other issues related to domestic terrorism. 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. Also possibly contributing to domestic terrorism's secondary status as a threat, 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 certainly 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, and there appears to be growth in anti-government extremist activity as measured by watchdog groups in the last several years. Much like their jihadist counterparts, domestic terrorists are often Internet savvy and use the medium as a resource for their operations. Prison has been highlighted as an arena that can foster terrorist radicalization, and white supremacy—a set of beliefs held by white supremacist extremists—has long played a role in the activities of several U.S. prison gangs. Sovereign citizen anti-government ideas (that have inspired some domestic terrorists) have also circulated in U.S. prisons. 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 opposition to 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, 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. 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 and concludes by examining potential issues for Congress. This report does not discuss in detail either violent jihadist-inspired terrorism or the federal government's role in counterterrorism investigations. It is meant to be read in conjunction with CRS Report R41780 , The Federal Bureau of Investigation and Terrorism Investigations ; CRS Report R42553 , Countering Violent Extremism in the United States ; and CRS Report R41416 , American Jihadist Terrorism: Combating a Complex Threat , which provide greater context in these areas. 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 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. 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, the Department of Justice (DOJ) has identified a number of general threats that embody this issue. 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 domestic terrorism 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. It is unclear exactly what the FBI means 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 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.