It seems to me that would put antifa in the terrorist camp.
Note there have been investigations following up concerning prior protests where lefty counter protestors have shown up to kick ass and the police have arrested the correct perpetrators.
So I agree and sympathize with your concern that an investigation could be abused to target the wrong folks, but it is not necessarily the case it'll work out that way.
If it is it'll be out there for all to see, just like competing investigations of Trump and Hilary.
Problem with this is that it means the United States is a terrorist nation whenever it intervenes with "violence" in order to effect political change, even if its to put in a new political system for the sake of national security concerns.
My definition is still better: Anything that Muslims do against Non-Muslims.
It seems to me that would put antifa in the terrorist camp.
They're terrorists since they're basically Muslim gorillas, except they throw poo and attack old ladies instead of suicide bombings. Soon they will begin using guns though. There will be blood on the streets before long, then it'll be our turn to, umm, "use violence for political ends" to sort out the problem.
Tongue in cheek aside, the car-ramming wasn't terrorism because there's no evidence he ran the car into ANTIFA because he wanted to vote out Virginia's Clintonista governor. There are better and more likely possibilities.