"It's not one specific organization with a headquarters and a president and a chain of command," said Mark Bray, a history professor at Rutgers University and author of "The Anti-Fascist Handbook." "It's a kind of politics. In a sense, there are plenty of antifa groups, but antifa itself is not a group."
That description is largely in line with how federal law enforcement has interpreted the antifa movement, leading up to this week's protests, which in certain instances have devolved into violence, looting and vandalism. Experts on antifa, which is shorthand for "anti-fascism," say the movement originates with groups that opposed World War II-era dictators like Italy's Benito Mussolini and Adolf Hitler.
While Attorney General William Barr in a statement Sunday denounced "violence instigated and carried out by antifa and other similar groups," the Justice Department as of Wednesday has not made public direct evidence showing widespread involvement by avowed antifa supporters in instigating the violent scenes that have unfolded throughout the U.S.
In the several federal cases brought thus far against those involved in riots or arsons, antifa has not yet been cited as among the affiliations or inspirations of the individuals charged.
In an appearance before the Senate last year, FBI Director Christopher Wray was pressed as to why federal agents haven't sought to mount an organized crime investigation of antifa related to its members' attacks at rallies and other alleged criminal acts.
"For us, antifa we view as more of an ideology than an organization," Wray said. "We have quite a number though, I should tell you, of properly predicated investigations of what we categorize as ‘anarchist extremists,' people who are trying to commit violent, criminal activity that violates federal law, and some of those people do subscribe to what we would describe as -- to what we would refer to as kind of an antifa-like ideology."
Though the FBI has said it is seeking information on "violent instigators who are exploiting legitimate, peaceful protests and engaging in violations of federal law," it has not issued a public statement since the start of the national unrest singling out antifa.
Antifa movements in the U.S. can be traced back as early as the 1970s, according to historians, but Bray writes in his book that the Anti-Racist Action (ARA) groups in the late 1980s and 1990s -- with their actions to directly confront racists, neo-Nazis and white supremacists and stymie their recruitment efforts -- were the primary precursors for antifa in its current iteration.
While antifa's political leanings are often described as "far-left," experts say members' radical views vary and can intersect with communism, socialism and anarchism.
Antifa has attracted more public attention in recent years both in the U.S. and abroad for its militant followers' provocations and, in some cases, violent attacks at political rallies and protests.
"Essentially there's this belief that the only way to prevent fascism from rising and taking charge is to combat those who support fascism, physically and often in the street," said Oren Segal, the vice president of the Center on Extremism at the Anti-Defamation League. "There's a lot of people who don't like Nazis and fascists -- most people I would argue. What makes [antifa] different is that there are some guiding principles, if you will, and that's the concept of, you know, a physical confrontation in order to combat a fascist."
A 2018 Congressional Research Service report outlined four "obligations" that antifa groups typically encourage of their followers, including, "(1) track the activity of fascist groups, (2) oppose their public organizing, (3) support antifascist allies attacked by fascists or arrested by police, and (4) not cooperate with law enforcement."
"Antifa followers tend not to accept that the conventional capacities of American society will thwart the rise of fascist movements," the report says. "They lack faith in the ability of federal, state, or local governments to properly investigate or prosecute fascists who break the law, especially during shows of force at public marches."
Perhaps the most famous confrontation came during 2017's Unite the Right rally in Charlottesville, Va., when violence erupted between white nationalists, neo-Nazis and counter protesters, which included some supporters of antifa.
President Trump's first public mention of antifa came just two weeks after the rally as he faced major backlash over his response to the protests and the killing of Heather Heyer by an avowed neo-Nazi.
"You know, they show up in the helmets and the black masks, and they've got clubs and they've got everything -- antifa!" Trump said during a campaign rally that month.
In the two years since, antifa supporters have engaged in similar skirmishes typically targeting far-right or alt-right protests at Berkeley, California, Stone Mountain, Georgia, and Portland, Oregon, among other cities.
But antifa supporters' activities are far from restricted to political rallies, Bray said.
"Most of what they do entails researching who far-right groups are, figuring out who their leaders are and notifying their employers and their communities," Bray said. "So it's a lot of a kind of private investigator work that sometimes spills out into the streets with confrontations."
Even as antifa has generated an increasing amount of attention and scrutiny in recent years, Bray said its lack of an official structure makes it difficult to estimate how many Americans it might count among its "members," who are mostly spread out across secretive local chapters.
"These groups tend to be very small, in large part because they're concerned about infiltration from law enforcement or the far right," Bray said. "Some groups don't even allow new members in order to prevent that, and the ones that do heavily vet the new members over the course of months and years to make sure that they have a full commitment, just to keep the numbers small."
Such an approach, though, has created opportunities that groups opposed to antifa can try to exploit. Just this week, Twitter moved to disable an account that appeared to represent a violent antifa group after it was determined to be a front account for a white nationalist group. Prior to being taken down, U.S. law enforcement had called out the group specifically as an example of a left-wing group trying to incite violence.
Bray said, however, that he believes that the president's efforts to try and attribute the violence and looting around the country almost exclusively to antifa vastly overstates both the membership numbers and capabilities of its followers.
"It seems like a rather transparent attempt for him to deflect away from the underlying white supremacy behind the murder of George Floyd," Bray said. "That's not to say that there aren't members of antifa groups participating in the various aspects of these protests, but their numbers are so minuscule nationally that they wouldn't even be able to logistically do nearly as much as he's blaming them for."