Thomas Matthew Crooks - Skytten mot Trump

A search by Fox News Digital revealed that Crooks would have turned 21 years old this September. He had no reported criminal or traffic citations and no reported bankruptcies, liens or foreclosures.

In terms of what led up to this ... Difrischia tells us he and his wife, Amber, were watching the Trump rally from outside the main grounds ... standing near some trees with about 30 to 40 other people, when Amber noticed the dude climbing up onto a nearby building there.

We're told he shimmied up, then army crawled to his final position on the roof -- all with a rifle in hand, mind you -- and then he lined up his shot, despite people clearly noticing him down below and attempting to flag cops or whoever else would listen, we're told.

Eventually he fired, and like we said ... he was met with almost instantaneous gunfire in return ... killing him. Others on the ground caught the aftermath, showing his dead body.