Non-Muslim extremists have carried out 19 such attacks since Sept. 11,
according to the latest count, compiled by David Sterman, a New America
program associate, and overseen by Peter Bergen, a terrorism expert. By
comparison, seven lethal attacks by Islamic militants have taken place
in the same period.

If such numbers are new to the public, they are familiar to police
officers. A survey to be published this week asked 382 police and
sheriff’s departments nationwide to rank the three biggest threats from
violent extremism in their jurisdiction. About 74 percent listed
antigovernment violence, while 39 percent listed “Al Qaeda-inspired”
violence, according to the researchers, Charles Kurzman of the
University of North Carolina and David Schanzer of Duke University.

http://www.nytimes.com/2015/06/25/us/tally-of-attacks-in-us-challenges-perceptions-of-top-terror-threat.html?_r=1