Well, antifa came straight out of the punk scene (remember Dead Kennedy's "nazi punks fuck off?) and before the modern incarnation began calling themselves antifa, they were known primarily as Anti-Racist Action and other groups such as FSU (Friends Stand United / Fuck Shit Up). Europe had a much stronger antifa scene historically as well. American antifa began to appear on the internet in the mid aughts on various websites such as myspace and xanga, primarily lead by the group One People's Project who from the beginning were FBI informants. I know this because I hacked their leaders' website and email in late 2007.

The FBI, since the Bush administration, was taking intelligence from anarcho-communist terrorists.