It doesn't appear that such authority exists unless the DOJ investigation can identify a definitive organizational structure with financial structure (bank accounts, etc).  Otherwise it is just 'a movement' (which is horseshit - a movement to usurp the democratic principles and framework the US was formed under and replace it with a global socialist structure).  What Barr has said is that Antifa's violent and destructive activity is domestic terrorism and will be treated as such.