If he cheered the destruction of the Twin Towers as a kid, that had to come from somewhere. It came from his family, and it came early in life.
He was born here, because he father was allowed to come here. He should not have been.
The system is not designed to catch people like this, in fact it seems designed to not catch people like this. They allowed his father to immigrate, he was allowed to work for a Homeland Security subcontractor, he was investigated by FBI who decided that suspicions about him were bigotry.
The system seems to be designed to never catch people like this - it turns out a gun shop turned him in for "odd requests/questions" and the Feebs did nothing....