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White supremacists and other far-right groups committed the majority of extremist-related murders in the United States last year, according to a new report by the Anti-Defamation League.

White supremacists were "directly responsible" for 18 out of 34 U.S. extremist-related deaths in 2017, the ADL said. Islamic extremists, by comparison, were only responsible for nine deaths in America.

Last year’s share of white supremacist killings represents a jump from 2016, when such groups accounted for nine homicides.

In an op-ed in The Atlantic accompanying the report, ADL National Director and CEO Jonathan Greenblatt noted that the uptick in white supremacist violence in 2017 occurred against the backdrop of an increasingly conspicuous alt-right movement.

A gathering of hundreds of white nationalists in Charlottesville, V.A., on August 12 Mark Peterson / Redux Pictures

"While it is impossible to draw a direct line of causation between these brutal acts and the many public displays of white supremacy that took place around the country in 2017,” Greenblatt wrote, "it’s critical not to underestimate the effect of an increasingly visible alt-right and white-supremacist community on people who are already predisposed to racism."

“As a society we need to keep a close watch on recruitment and rallies such as Charlottesville, which have the greatest potential to provoke and inspire violence,” Greenblatt wrote, referring to last summer’s white nationalist rally, when a counter-protestor was killed.

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While white supremacists accounted for the largest share of killings last year, an Islamic extremist is charged with committing the single deadliest incident: Sayfullo Saipov allegedly rammed into pedestrians in New York City in October, killing eight.

Last year’s total of 34 extremist-related fatalities represents a sharp decline from the previous two years – though it was still the fifth-deadliest year since 1970, according to the ADL. The organization recorded 71 extremist-related killings in 2016 and 69 in 2015.

The statistics were culled from the ADL’s own tracking of extremist crimes. The organization said that its statistics would likely be revised upwards, as extremist connections to some murders can take months to uncover.

But different organizations track extremist violence in different ways. The ADL counts non-ideological murders committed by extremists in its tally — so a dedicated white supremacist who commits a murder unrelated to his political beliefs would be counted, for example. New America, a think tank with its own tracking system, excludes such statistics.

“For us, there has to be clear causation, not just correlation,” said Albert Ford, a New America researcher.

New America counted five murders by far-right extremists in 2017, nine by Islamic extremists, and three by black nationalists.

Either way, extremist killings represent only a fraction of the total homicides in the U.S., though they tend to garner more media coverage and have an outsize impact on public debate.

Such was the case with Heather Heyer, the counter-protestor killed by James Alex Fields Jr. at last summer’s “Unite the Right” rally in Charlottesville. Her death ignited a national discussion about far-right groups.

The ADL tallied five murders by black nationalists in 2017, following similar violent incidents in 2016 and 2014 — which it noted as “the most significant black nationalist-related violence since the early 1980s” and “a possible emerging extremist threat.“

An absence of large shooting sprees explains last year’s lower extremist-linked murder tally, according to the ADL. In 2016, Omar Mateen killed 49 people at the Pulse nightclub in Orlando, Florida. In 2015, white supremacists and Islamic extremists engaged in three significant shooting sprees that left 28 dead.

The deadliest extremist-related incident in 2017 involved neither firearms nor explosives – a departure from past years. The New York attack was carried out by truck. The deadliest shooting incident last year took place in Fresno, Calif., and resulted in four deaths.

Last year’s data match historical trends recorded by the ADL. The organization counted 387 extremist-related murders over the last decade: Right-wing extremists were responsible for 71 percent of those murders, while Islamic extremists were linked to 26 percent.