Sounds like the kind of stuff the Feebs are supposed to know before someone goes off the rails. And maybe they did...and recruited, or encouraged him. All of the nutty internet posts in the world won’t get you easy access to a rooftop overlooking a DJT event, to say nothing of taking a rifle with you.
The point is they did know about it.
Not doing anything about it means they let him go off when he had the opportunity.
It’s called being Serpico’d. You just stand back and let the ghouls do what they do. Then pretend you didn’t see it coming.