Antifa is an umbrella term to describe radical left-leaning militant groups that typically confront neo-Nazism and white supremacists at demonstrations.

Public and elected officials, including President Trump, have blamed antifa activists for violence at protests sparked by the death of George Floyd last week. Floyd, a 46-year-old black man, died last Monday after a white Minneapolis police officer pressed his knee on Floyd's neck for several minutes.

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But officials have said thatin parts of the country, what started as peaceful protests have devolved into organized riots.

Short for anti-fascists, antifa has no hierarchical structure or universal set of tactics that makes its presence immediately recognizable, though members tend to espouse revolutionary and anti-authoritarian views, said Mark Bray, a historian at Rutgers University and author of Antifa: The Anti-Fascist Handbook.

Literature from the antifa movement encourages followers to pursue lawful protest activity as well as more confrontational acts, according to a 2018 Congressional Research Service report.

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The literature suggests that followers monitor the activities of white supremacist groups, publicize online the personal information of perceived enemies, develop self-defense training regimens and compel outside organizations to cancel any speakers or events with a fascist bent, the report said.

People associated with Antifa have been present for significant demonstrations and counter-demonstrations over the last three years, sometimes involving brawls and property damage.

Trump and members of his administration have singled out antifa as being responsible for the violence at protests triggered by the killing of Floyd.

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On Tuesday, Trumps personal attorney, Rudy Giuliani, told FOX Business Maria Bartiromo the FBI has evidence that shows antifa's involvement.

This is an organized effort. I dont say its a strictly organized effort, but its organized, Giuliani said. Theyre communicating with each other, you know the FBI has a lot of the texts that show antifa's involvement. Theyre anarchists, theyre people who want to drive this government down.

In a pair of statements over the weekend, Attorney General William Barr described antifa-like tactics" by out-of-state agitators and said antifa was instigating violence and engaging in domestic terrorism" and would be dealt with accordingly.

At a White House appearance Monday, Trump blamed antifa by name for the violence, along with violent mobs, arsonists and looters.

White House press secretary Kayleigh McEnany told reporters earlier in the day that antifa is a big element of this protest," though she deferred to the Justice Department on the question of how one could be identified as a member.

But it's unclear how big its involvement is.

Police stand near an overturned vehicle and a fire as demonstrators protest the death of George Floyd, Sunday, May 31, 2020, near the White House in Washington.(AP Photo/Alex Brandon)

Bray said that although he believes people associated with antifa are participating in the demonstrations, it is difficult to establish how big of a role they're playing since there is no official roster of members and since the movement lacks the numbers to mobilize nationwide in such a dramatic, forceful way.

The radical left is much bigger than antifa much, much bigger and the number of people who are participating in the property destruction are much, much bigger than the radical left, Bray said.

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Little is known about who funds antifa activists, or how the groups get their resources. Antifa is not a single organization, and therefore, financial details, if any exist, are murky.

Earlier this week, Trump tweeted: The United States of America will be designating ANTIFA as a Terrorist Organization."

Asked Monday what legal authority the president would have for labeling antifa a terror organization, McEnany pointed to the existing statute under the U.S. criminal code that defines acts of domestic and international terrorism.

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Even if antifa is not a designated terror organization, FBI Director Chris Wray has made clear that it's on the radar of federal law enforcement.

He has said that while the FBI does not investigate on the basis of ideology, agents have pursued investigations across the country against people motivated to commit crimes and acts of violence "on kind of an antifa ideology."

The Associated Press contributed to this report.

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What is antifa and who funds it? | Fox Business