Easy. He is one of theirs. Or they had lots of time to ID him as they gaped at him while he took aim. They have stuff on all of us, who knows how fast their digital system works.
At first I thought damn! an Antifa crazy. Then all the apparent negligence was reported and it began to look like a hired gun, or rather an impressionable antifa crazy that got some carefully planted suggestions from a federal agent. Such a one can be killed at the site to cover tracks.