A May 15 article by J Oliver Conroy in The Guardian explains why to many in law enforcement, the biggest terror threat in the U.S. is not from radical Muslims but fanatical anti-government activists, many but not all on the remote fringes of the far right.
Conroy gives a series of graphic examples of politically-motivated attacks on law enforcement officers, often by self-styled "sovereign citizens" who believe themselves essentially above the law. Fortunately the total number of these attacks is not that large, at least in comparison with the overall murder rate, but the breakdown might come as a surprise to some. Quoting Conroy's article,
According to data from the Anti-Defamation League, at least 45 police officers have been killed by domestic extremists since 2001. Of these, 10 were killed by leftwing extremists, 34 by rightwing extremists, and one by homegrown Islamist extremists.
The pattern is roughly similar for violent extremist acts directed at the public:
In fact, a 2016 report by the US Government Accountability Office noted that "of the 85 violent extremist incidents that resulted in death since September 12, 2001, far-rightwing violent extremist groups were responsible for 62 (73%) while radical Islamist violent extremists were responsible for 23 (27%)." (The report counts the 15 Beltway sniper shootings in 2002 as radical Islamist attacks, though the perpetrators’ motives are debated.)
The second-most-deadly terrorist attack in American history, the Oklahoma City bombing that killed 168 (including 19 children) in 1995, was an earlier example of far-right anti-government domestic terrorism. One of the men convicted for the murders called himself a "sovereign citizen."