President Donald Trump has blamed antifa activists for violence at protests over police killings of black people, but antifa isn't an organisation and targeting it isn't simple.
A look at what antifa is and is not:
WHAT IS ANTIFA?
Short for "anti-fascists", antifa is not a single organisation but rather an umbrella term for far-left-leaning militant groups that confront or resist neo-Nazis and white supremacists at demonstrations.
There is no hierarchical structure to antifa or universal set of tactics that makes its presence immediately recognisable, 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.
"They do different things at different times in different ways, some of which there is evidence of them breaking the law. Other times there is not," Bray said.
Literature from the antifa movement encourages followers to monitor the activities of white supremacist groups, publicise the personal information of perceived enemies, develop self-defence training regimens and compel outside organisations to cancel any speakers or events with "a fascist bent", according to a 2018 Congressional Research Service report.
People associated with antifa have been present for significant demonstrations and counterdemonstrations over the past three years, sometimes involving brawls and property damage.
They mobilised against a white supremacist march in Charlottesville, Virginia, in August 2017 and have clashed repeatedly with far-right groups in Portland, Oregon, including at a rally last summer that resulted in arrests and the seizure of shields, poles and other weapons.
WHAT ROLE IS ANTIFA PLAYING IN THESE DEMONSTRATIONS?
Trump and members of his administration have singled out antifa as being responsible for the violence at protests triggered by the killing of George Floyd, a black man who died after a white Minneapolis police officer pressed his knee into his neck for several minutes even after Floyd stopped moving and pleading for air.
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.
White House press secretary Kayleigh McEnany told reporters Monday that antifa is a "big element of this protest", though 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.
Bray said that although he believes people associated with antifa are participating in the demonstrations, it is difficult to determine how big of a role they're playing since there is no membership uniform or calling card and since the movement lacks the numbers to mobilise 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.
Others have seen evidence of right-wing extremists.
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WHAT DOES THE TRUMP ADMINISTRATION WANT TO DO ABOUT ANTIFA?
Trump tweeted Sunday, "The United States of America will be designating ANTIFA as a Terrorist Organisation."
It's not the first time he's endorsed that approach. Trump tweeted a similar sentiment last summer, joining some Republican lawmakers who called for antifa to be designated as a terror organisation following the skirmishes in Portland.
CAN THE ADMINISTRATION DO THAT?
For one thing, antifa is not a discrete or centralised group, so it's unclear how the government could give it a designation.
Beyond that, though, antifa is a domestic entity and, as such, not a candidate for inclusion on the State Department's list of foreign terror organisations. Those groups, which include Islamic extremist organisations and the Real Irish Republican Army, are based overseas rather than in the US.
That designation matters for a variety of legal reasons, not least of which anyone in the United States who lends support to an organisation on that list can face terrorism-related charges.
But "there is not a domestic equivalent", said Joshua Geltzer, a former senior counterterrorism official in the Obama White House and founding executive director at the Institute for Constitutional Advocacy and Protection at the Georgetown University Law Centre.
There have been periodic calls, particularly after mass shootings by white supremacists, to establish a domestic terrorism law, but none has passed.
Asked Monday what legal authority the President would have for labelling antifa a terror organisation, McEnany pointed to the existing statute under the US criminal code that defines acts of domestic and international terrorism.
But defining an act of terrorism is different than designating an entire group as a terror organisation.
"US law does the 1st. It doesn't permit the 2nd," Geltzer tweeted after McEnany's remarks.
Even if antifa is not a designated terror organisation, 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 acts of violence "on kind of an antifa ideology".