2015 was America's deadliest year for domestic extremist killings since the Oklahoma City bombing

This past year was the deadliest year for domestic extremism-related shootings in 20 years, according to a new report from the Anti-Defamation League.

The report accounted for 52 people who were killed by domestic extremists, which is more than the total from 2013 and 2014 combined. It was the most deaths from domestic extremism since 1995, when Timothy McVeigh and Terry Nichols bombed the Alfred P. Murrah Federal Building in Oklahoma City, killing 168.

The greatest percentage of domestic extremist killings came at the hands of white supremacists, accounting for 38% of those deaths. Nearly half of those killings oc cured in the June 17 mass shooting at the Emanuel AME Church in Charleston, South Carolina.

Domestic Islamic extremism made up the next-highest percentage, accounting for 37% of the killings in 2015. The December 2 attack by Syed Farook and his wife Tashfeen Malik at the Inland Regional Center in San Bernardino, California - which was the second-deadliest instance of domestic extremism-related killing since 1966 per ADL data - and the July 16 shooting by Muhammad Abdulazeez at a military recruitment center in Chattanooga accounted for those killings.

The remaining killings were allegedly carried out by Robert Deer - an anti-abortion extremist - in Colorado Springs, and various anti-government extremists.

Looking at domestic extremism-related killings throughout the past decade, during which the ADL has attributed 295 deaths to such causes, 70% have been committed by white supremicists, while 13% have come at the hands of domestic Islamic extremists.

The ADL does mention that its data isn't perfect, since police aren't able to identify every extremist-related killing and some killings that may be considered extremist-related by some aren't always classified as such.

But it is clear that, overwhelmingly, incidents of extremist-related killings are carried out with firearms, the group said.

"The blunt fact is that, in the past 50 years, firearms in the hands of domestic extremists have killed far more Americans than have bombs, blades, chemical or biological weapons, or any other type of weapon," the report stated. "Right-wing extremists have long known the ease and efficiency of firearms as a weapon for terrorist and extremist violence; domestic Islamic extremists may unfortunately be catching on. The combination is likely in the future to add still further to the toll of life taken by domestic extremists in the United States."

Recognizing the toll taken by gun violence, President Barack Obama teared up on Tuesday while announcing a slate of executive actions on gun control.