Feds now tracking 850 possible domestic terrorists across US, as white supremacy cases jump: Officials The head of the FBI's Counterterrorism Division testified Wednesday to Congress.

The FBI is currently investigating nearly 850 people across the United States as possible domestic terrorists, and the number of cases targeting white supremacists, white nationalists and other racially-motivated extremists has jumped in the past six months, federal law enforcement officials said Wednesday.

“In fact, there have been more arrests and deaths in the United States caused by domestic terrorists than international terrorists in recent years,” the head of the FBI's counterterrorism division, Assistant Director Michael McGarrity, told a House panel Wednesday.

He warned that hatred and extremist ideology are increasingly being spread online, and “that mobilization to violence is much quicker” than it used to be. McGarrity emphasized that "domestic terrorism" -- often rooted in racially-motivated extremism, anti-government extremism or environmental extremism -- is different than "homegrown" terrorism, which is inspired by international terrorist groups like ISIS and Al Qaeda.

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Less than two weeks ago, a 19-year-old California man allegedly launched a deadly attack on a synagogue outside San Diego, and authorities suspect he left behind a message online touting white supremacy and anti-semitism. Months earlier, another assault on a synagogue in Pittsburgh left 11 people dead, and it was allegedly launched by another extremist believed to have been radicalized online.

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"You can go on the internet and find content that justifies what you want to do," McGarrity noted during testimony to the House Homeland Security Committee.

Nevertheless, U.S. authorities are seeing “a tide change” in many social media companies starting to police their own content, according to McGarrity.

He said that of the 850 domestic terrorism investigations currently underway inside the United States, about 40 percent target subjects who adhere to racist ideologies, and “a significant majority” of them are white nationalists or white supremacists. The vast majority of the other cases involve subjects who promote anti-government or anti-authority sentiments, McGarrity testified.

He also told lawmakers that the number of current domestic terrorism investigations is actually down from six months ago, McGarrity said. And in September 2017, FBI Director Chris Wray testified that -- at that time -- the FBI was conducting about 1,000 domestic terrorism investigations.

But on Wednesday, a federal law enforcement official told ABC News that -- even though the overall number of domestic terrorism investigations has dropped in the past six months -- the number of domestic terrorism investigations targeting white supremacists, white nationalists and other racially-motivated individuals has actually gone up in the same time period.

During his House testimony, McGarrity said “the velocity” of domestic terrorism cases “is much quicker than it’s ever been.” Like ISIS-inspired terrorists, domestic terrorists are being radicalized much faster than ever before thanks to the internet, and that makes it much harder for authorities to detect them, McGarrity said.

McGarrity said the "most deadly" domestic terrorism threat facing the country is "the lone offender who self-radicalized online [and] who has access to a weapon.”

However, McGarrity said, “We are seeing a tide change in social media companies being more proactive, policing their own."

A senior official from the Department of Homeland Security's Office of Intelligence and Analysis, Brian Murphy, seemed to agree, saying, “We are making strides, and we would like to keep making strides.”

But Murphy indicated that not all social media companies are as willing to police their own content as some of their competitors.