The Anti-Defamation League (ADL) is warning of an increase in violence by white supremacists who the group says have become "emboldened" by the rise of the alt-right.

Eighteen people were reportedly killed in white supremacist violence in 2017, the ADL reported — more than double the previous year’s total of seven

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Overall, the ADL recorded 34 killings by extremists in the United States, a tally that includes eight people killed by an Islamic extremist who veered his truck into a New York City bikepath. A note at the attack claimed responsibility by the Islamic State in Iraq and Syria.

The number of people killed by extremists dropped sharply from 2015 and 2016, though the ADL said it was still the fifth deadliest year of extremist violence in the United States since 1970.

In 2016, the numbers were sharply higher in part because of the shooting at the Pulse nightclub in Orlando, where 49 people were killed by a man who identified with Islamic terrorists. It was the deadliest shooting in modern U.S. history at that time.

Extremist-related killings represent a tiny portion of homicides in the United States, the ADL said. But it argued they are important because of the way they can effect entire communities.

As an example, the report highlights the death of Heather Heyer, a young woman killed when a man, photographed attending a white supremacist rally, is accused of later driving his car into a crowd of people marching in protest of racism.

While people tied to Islamic extremism have been responsible for mass shootings in Orlando and in San Bernardino, Calif., in 2015, the ADL said that domestic right-wing extremists were responsible for 71 percent of U.S. fatalities from extremist violence in the past decade. Twenty-six percent of killings in the U.S. in that timeframe were by Islamic extremists, it said. The other three percent were by extremists that did not fit either of those categories.

As a result, ADL CEO Jonathan Greenblatt said it would be a mistake to ignore domestic terrorism and only focus on Islamic extremists.

“These findings are a stark reminder that domestic extremism is a serious threat to our safety and security,” he said. “The bottom line is we cannot ignore one form of extremism over another. We must tackle them all.”