If you read the Politico article that you link, FBI, Homeland Security warn of more ‘antifa’ attacks, it says:
Previously unreported documents disclose that by April 2016, authorities believed that “anarchist extremists” were the primary instigators of violence at public rallies against a range of targets. They were blamed by authorities for attacks on the police, government and political institutions, along with symbols of “the capitalist system,” racism, social injustice and fascism, according to a confidential 2016 joint intelligence assessment by DHS and the FBI.
said one senior law enforcement official tracking domestic extremists in a state that has become a front line in clashes between the groups. “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 Atlantic Monthly reports:
A similar cycle has played out at UC Berkeley. In February, masked antifascists broke store windows and hurled Molotov cocktails and rocks at police during a rally against the planned speech by Yiannopoulos.
As members of a largely anarchist movement, antifascists
Adherents are mostly socialists, anarchists, and communists who, according to Mark Bray, a historian at Dartmouth College and author of Antifa: The Anti-Fascist Handbook
Some things that we can take from these descriptions.
- Antifa is not monolithic but is instead composed of multiple ideologies that are only loosely joined. They are all "against fascists" but their means can be very different.
- It is generally acknowledged that one of those ideologies is anarchism.
- That some people who have supported or engaged in violence have claimed to be members of antifa.
In Charlottesville, it is undisputed that the neonazis engaged in violence, including one murder. So it could be argued that counterviolence was self defense or at least heat of the moment.
In Berkeley however, the vandalism was not a response to any violence from Yiannopoulos but an attempt to keep him from speaking.
It is unclear if the FBI is using the term anarchists to refer to antifa. That characterization is common in the media. It is possible that Politico added it.