"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.' […] 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 [Congressional Research Service] 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."
CRS Report for Congress, R42536