The number of killings carried out by white supremacists in the U.S. more than doubled from 2016 to 2017, according to a watchdog's report. The Anti-Defamation League said this week that white supremacists were directly responsible for 18 of the 34 extremist-related killings in 2017.

The group said Islamic extremists were linked to nine deaths in the U.S. last year, eight of which came from a van attack on a New York City bike path on Halloween.

Extremists carried out fewer killings in the U.S. in 2017 than in 2016 or 2015, which saw 71 and 69 such deaths respectively, the group said. Still, when compared to every year since 1970, 2017 ranks as the fifth-deadliest for extremist violence, the group said.

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"These findings are a stark reminder that domestic extremism is a serious threat to our safety and security," the group's CEO Jonathan Greenblatt said in a statement. "We saw two car-ramming attacks in the U.S. last year -- one from an Islamic terrorist and another from a white supremacist in Charlottesville -- and the number of deaths attributed to white supremacists increased substantially. The bottom line is we cannot ignore one form of extremism over another. We must tackle them all."

The group's report comes on the heels of a new study by the departments of Homeland Security and Justice. They found that 73 percent of people convicted in U.S. federal courts on charges related to international terrorism between Sept. 11, 2001, and the end of 2016 were born outside the U.S.

The study also found that 148 of those people had become naturalized U.S. citizens.

"It's very concerning," Secretary of Homeland Security Kirstjen Nielsen told "CBS This Morning" co-host John Dickerson in an interview broadcast this week. "I think what it underlines is that we need to be very clear who's coming into our country, make sure they're not a terrorist."

The study also found that another 147 people convicted of international terrorism-related charges during that period were U.S.-born citizens.

"We need to do more to prevent radicalization," Nielsen told Dickerson, "the inspiration of terrorism in this country."