White supremacists and anti-government radicals have accounted for almost twice the number murders in the United States than Muslim jihadis in the years following the September 11 2001 terror attacks, a recent Washington research center study revealed.

In the span of 15 years, anti-government radicals and white supremacists have reportedly killed 48 Americans, according to a count by New America. The number by far exceeded the 26 deaths caused by Muslim radicals in the same period.

The research team’s program associate David Sterman concluded that along with the Muslim extremists, white supremacy and anti-government idealists also pose a significant threat to the American public, highlighting the “ignored threat” that has grown within American society, the Washington Times reported.

“Each time it [right-wing, radical violence] comes up, there’s a tendency to dismiss it as lone actor, mental health issues,” Sterman was quoted as saying. “So it’s important to not ignore threats.”

The report came a week after 21-year-old Dylann Roof, the suspect of a deadly attack inside a Charleston church that killed nine people, posted his racist manifesto online. Roof, who is now facing charges of federal hate crime, also posted photos with white supremacist and Nazi symbolism.

The study also pointed out how targets vary between groups. While most Muslim extremist attacks were usually focused on military targets and public areas where mass casualties are sought, right-wing radicals target mostly police operations, promoting anti-government sentiment, New America said.

Observers believe that the rise in white supremacist attacks may be due to the U.S. government’s focus on Islamic extremists and potential domestic attacks, leaving right-wing extremists out of the spotlight.

Another theory, posited by The Heritage Foundation senior legal fellow John Malcolm, points out that the low Islamic extremist attacks may in fact be due to the government being able to thwart terror plans before they even happen.

Sterman agreed, adding that in describing terrorism, acts should not be labeled whether done by a Muslim or non-Muslim. He stated that if there are indications that a suspected plot is not linked to Islamic State, it should simply be ignored.

“For example, in the Dylann Roof case, in the Charleston attack, you do see that he is leaking quite a bit of information to people around him about his viewpoint and his desire to commit violence,” said Mr. Sterman. He also pointed out that if it was an attack by Islamic extremists, federal authorities would have their attention on it until the threat is contained.