The Anti-Defamation League has for decades has tracked anti-Semitic and other hate crimes in the United States. In a report released 2019 January 23 (summarized here) the organization's Center on Extremism compiles the latest statistics on murders committed by persons connected with domestic extremist groups and movements in the United States. Most of these murders also take place inside the United States, though there have been exceptions.
Note that this focus excludes murders committed in the United States by foreign members of foreign terrorist groups, the most obvious example being September 11 al Qaida attacks, the most deadly terrorist incident in history. However, various other things I've read suggest that both before and since 9-11, the great majority of attacks in the United States motivated by extremism have been carried out by U.S. residents tied to home-grown extremism, even when the extremism is part of a wider-spread movement.
Three things in the report strike me as particularly interesting.
One is that during the 10-year period 2009-2018, there were 427 murders attributed to violent extremism. The report notes that as additional information becomes available, the report will be updated, so the number of murders is likely to increase some over time, especially those for the most recent years. However, the total isn't likely to change much. This isn't to minimize the danger or dismiss the pain caused to the victims and their friends and families, nor to suggest that law enforcement shouldn't continue to try to prevent terrorism (such as the violence apparently contemplated by an extremist member of the Coast Guard). Still, it's worth recognizing that at present it's nowhere near the greatest danger we face.
Also, of those 427 murders, 313 (73.3%) were carried out by persons associated with what the report classifies as right-wing extremism. A number of years ago a number of conservative pundits and politicians objected strongly to a Justice Department report on the danger of right-wing violence (perhaps because they didn't realize there was another one about the threat of left-wing violence). These numbers suggest that if one is concerned about domestic terrorism at all, extremist ideologies on the right ought to be taken into account.
Of the the 313 murders attributed to right-wing extremism, 76 percent were tied to white supremacy, 19 to anti-government extremism, 3 percent to the "involuntary celibacy" (or "incel") movement -- basically guys pissed off because the women they want won't have sex with them -- 1 percent to anti-abortion extremism, and the remainder to other right-wing causes. One could of course quibble with the classifications. Very few of those on the far right identify with the "incel" movement, for example, though on the other hand people involved in "incel" appear to be far-right in their other views. Clearly they're not fans of feminism.
It's of course also important to recognize that mainstream conservatives or liberals are pretty much by definition not extremists, despite the tendency of many on both sides to label members of the other as "far-right" or "far-left."
U.S.-based Islamist extremism accounted for 23.4 percent of the 427 extremist-affiliated murders over those ten years. Left-wing extremism, a major basis for terrorism decades ago in Europe and other parts of the world, accounted for the remaining 3.2 percent.
Finally, of the 50 murders last year (that is, 2018) attributed to extremism in the U.S., only 1 death was attributed to Islamic extremism. Of the rest, 39 were tied to white supremacy, 8 to anti-government extremism, and 2 to "incel." Interestingly, the killer deemed motivated mainly by Islamic extremism apparently also had some interest in white supremacy. Of course, any single year's data can be potentially misleading and longer-term numbers are probably more reliable as a gauge of where the dangers actually lie. Again, though, these dangers appear to be pretty small in comparison with the many other causes of violence.