(CNN) - White supremacists were responsible for the majority of extremist killings in 2017 compared to other groups, according to a newly released report by the Anti-Defamation League.
Of the 34 people the league's Center on Extremism found were killed by domestic extremists last year, right-wing extremists killed 20 people, with 18 of those killed by white supremacists, it said in the report released Wednesday.
"Extremism in any form is an issue. Foreign born, politically minded extremism or racially focused extremism. What the data tells us in the past 10-plus years it is far right-wing extremism, white supremacists and their ilk that are responsible for more extremist-related murders than any other group," said Richard Greenblatt, CEO of the Anti-Defamation League.
Last year was the fifth deadliest for extremist violence since 1970, when the ADL began tracking these types of crimes.
"Concretely it confirms extremists feel emboldened in the current environment. Right-wing extremists in particular were responsible for nearly 60% of extremist-related fatalities last year," Greenblatt said, "The data lays bare that this is not an exaggeration, as some would try to paint it."
The ADL report includes killings that have not yet gone through the legal system. Some of those shook America's consciousness, including the deaths of Heather Heyer in Charlottesville, Virginia, and Ricky John Best and Taliesin Myrddin Namkai-Meche in May in Portland, Oregon.
Heyer, 32, died after a car plowed into a crowd of protesters gathered to oppose a "Unite the Right" rally of white nationalist and other right-wing groups last summer.
Police say a man yelling racial and religious epithets stabbed Best, 53, and Namkai-Meche, 23, to death on a Portland train after they defended a young black woman and another woman wearing a hijab.
Bewilderment over DHS actions
The Anti-Defamation League's annual report comes a day after Homeland Security Secretary Kirstjen Nielsen spoke about the threats to America to the Senate Judiciary Committee. She didn't mention the threat of right-wing extremist violence in the country.
"DHS has historically had a focus on right-wing extremism. It makes no sense not to acknowledge those groups that committed the preponderance of the extremist violence and murder in this country," Greenblatt told CNN.
California Democratic Sen. Kamala Harris also noted the omission, tweeting: "It is deeply troubling that when talking about threats to our nation, Secretary Nielsen failed to mention a report that talks about some of the most rampant terror attacks that face our nation -- domestic acts of terror including white supremacist extremists."
Extremist killings down sharply from 2016
While the killings by extremists generally make up a tiny fraction of the number of murders in the United States, their effect can often have a huge impact on entire communities, the ADL report said.
Unlike 2016, which saw the killing of 49 people in the Pulse Nightclub in Orlando by self-described Islamic soldier Omar Mateen, 2017 saw a steep drop in killings by extremists, but it was right-wing extremists who committed the majority of killings, with 20 of the 34, the ADL report said.
The report also noted, "Extremists from a variety of different movements were involved in murders in 2017, including Islamic extremists, black nationalists, as well as members of the alt-right." The report said nine of the 34 killings in 2017 were done by those the ADL classified as Islamic extremists.
The ADL also found what could be another disturbing trend.
"2017 was the second year in a row where black nationalists committed murders. There were five total. It suggested there could be an emerging problem," Greenblatt said.
The deadliest single attack occurred in New York on October 31 by accused Islamist extremist Sayfullo Saipov. He is accused of driving a rental truck down a bike path and killing eight people.
"If we are going to solve this problem, extremism of any kind must be taken seriously," Greenblatt said.
Last October's deadly shooting in Las Vegas, which claimed the lives of 58 people, is not reflected in the ADL report, because the motive of the gunman remains unknown.
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