The FBI and Department of Homeland Security have been warning state and local authorities since early 2016 that the radical leftist movement known as "antifa" had become increasingly violent, describing its members as "anarchist extremists."

In fact, DHS formally classified their activities as "domestic terrorist violence" during the final year of the former Obama administration, Politico reported Friday, citing law enforcement authorities and confidential documents.

The FBI and DHS described antifa activists, who they called "anarchist extremists," as the primary instigators of violence at public rallies against a wide range of targets, including the police and government institutions, as well as racism, fascism, and "the capitalist system," according to a confidential 2016 joint intelligence assessment by both agencies.

Since the 2016 presidential election, antifa groups have directed their attention and violence towards supporters of President Donald Trump and white nationalist groups.

"It was in that period [as the Trump campaign emerged] that we really became aware of them," one senior law enforcement official who tracks domestic extremists told Politico. "These Antifa guys were showing up with weapons, shields, and bike helmets and just beating the shit out of people. … They're using Molotov cocktails, they're starting fires, they're throwing bombs and smashing windows."

The events in Charlottesville, Va. earlier this month have increased recruitment for both sides of extremists—antifa and white nationalists.

"Everybody is wondering, ‘What are we gonna do? How are we gonna deal with this?'" the senior state law enforcement official said. "Every time they have one of these protests where both sides are bringing guns, there are sphincters tightening in my world. Emotions get high, and fingers get twitchy on the trigger."

The April 2016 joint DHS-FBI report was titled "Baseline Comparison of U.S. and Foreign Anarchist Extremist Movements," and indicated that these groups spend weeks planning for violence at upcoming events.

Some Antifa activists have gone overseas to train and fight with other anarchist groups, Politico noted.

"There's a lot more we don't know about these groups than what we do know about them," said one senior New Jersey law enforcement official who follows antifa groups.