Fifty-two people died at the hands of domestic extremists in 2015, according to a report by the Anti-Defamation League's Center on Extremism. The figure is higher than that for the two previous years combined.

"The killers came from four extremist movements-white supremacists, domestic Islamic extremists, right-wing anti-government extremists, and anti-abortion extremists-illustrating the multiple threats posed by radical movements within America's borders," the report said.

The one year that surpassed 2015 was 1995, a year that included the Oklahoma City Bombings.

"The 52 murders documented by the Center on Extremism in 2015 occurred in 17 separate incidents, with nine of the incidents involving multiple murders. This is unusual, in that most extremist-related examples of murder involve a single victim," the report added. 

"Ideology played a primary or substantial role in 10 of the 17 incidents ... Typically, white supremacists make up the vast majority of non-ideological perpetrators."

The ADL does not overtly define extremism in their report or on their website. "At the root of extremism are radical ideologies, radical religious beliefs and pent-up anger and frustration, all of which can lead to violent acts ranging from hate crimes to terrorism," an archived ADL database notes. 

When the ADL speaks of non-ideological violence, they refer to crimes that were committed by extremists without extremist motivations.

Following an established trend, white supremacists have proven the deadliest, as they have in years past, with 20 murders in 2015. Marking a distinct shift, however, they were trailed closely by domestic Islamic extremists, who killed 19. 

In the years 2006-2015, white supremacists have been responsible for 70 percent of extremist murders, while domestic Islamic extremists have comprised only 13 percent, according to the ADL.

Anti-immigration extremists and left-wing extremists/anarchists- groups who have killed in recent years- did not kill any in 2015.