In the 14 years since Al Qaeda carried out attacks on New York and the Pentagon, extremists have regularly executed smaller lethal assaults in the United States, explaining their motives in online manifestoes or social media rants.

But the breakdown of extremist ideologies behind those attacks may come as a surprise. Since Sept. 11, 2001, nearly twice as many people have been killed by white supremacists, antigovernment fanatics and other non-Muslim extremists than by radical Muslims: 48 have been killed by extremists who are not Muslim, including the recent mass killing in Charleston, S.C., compared with 26 by self-proclaimed jihadists, according to a count by New America, a Washington research center.

The slaying of nine African-Americans in a Charleston church last week, with an avowed white supremacist charged with their murders, was a particularly savage case.