Shooter has been identified as Thomas Matthew Crooks. Some sources say he was affiliated with some anti-fascist group, and other places are saying he was a registered Republican.
So likely just a registered Republican. Antifa gets thrown onto literally every one of these types of people until proved otherwise. Remember the antifa utility attacks that turned out to be white supremacists?
He is a registered Republican (middle initial, and date of birth included).
Others claim he donated to a progressive run, Voter Turnout campaign push, but that donations on has a firat and last name listed so may not be this particular Thomas Crooks.
Other than that and his choice of weapon, there's not much to go on.