I thought that there really wasn't an "Antifa" as such, that anyone could call themselves or their organization "Antifa". But if there is an "Antifa", and it's

I thought that there really wasn't an "Antifa" as such, that anyone could call themselves or their organization "Antifa". But if there is an "Antifa", and it's made up of US citizens, under what law are they terrorists? Again, I may be misinformed, but I had understood "terrorist" as a legal designation was for non-US-citizens.