WASHINGTON — Nine people were added to a long list of lives taken by domestic terrorism when a 21-year old allegedly began shooting inside a historic Black church in Charleston, South Carolina, on June 17.

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, compared with 26 by self-proclaimed jihadists, according to a count by New America, a Washington research center.

The study found that radical anti-government groups or white supremacists were responsible for most of the terror attacks.

The data counters many conventional thoughts on what terrorism is and isn’t.

Since Sept. 11, many Americans attribute terror attacks to Islamic extremists instead of those in the right wing. But the numbers don’t back up this popular conception, said Charles Kurzman, a professor at the University of North Carolina at Chapel Hill. Kurzman is co-authoring a study with David Schanzer of Duke University that asks police departments to rank the three biggest threats from violent extremism in their jurisdiction.

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.

“Muslim extremism was taken seriously in many of these jurisdictions that we surveyed… but overall, they did not see as much of an issue with Muslim extremism as with right-wing extremism in their locations,” Kurzman told The Huffington Post.

America’s first federal anti-terrorism law, known as the Third Force Act or the Ku Klux Klan Act, which was passed by Congress in 1871, caused nine counties in South Carolina to be placed under martial law and led to thousands of arrests. The Supreme Court ruled the law unconstitutional in 1882.

On several occasions since President Obama took office, efforts by government agencies to conduct research on right-wing extremism have run into resistance from Republicans, who suspected an attempt to smear conservatives.

A 2009 report by the Department of Homeland Security, which warned that an ailing economy and the election of the first Black president might prompt a violent reaction from W hite supremacists, was withdrawn in the face of conservative criticism.

Mass killings in which no ideological motive is evident, such the Colorado movie theater incident and the Connecticut elementary school shooting in 2012, were excluded from the criteria used by New America.