Thomas Matthew Crooks took in the lay of the land at the Donald Trump rally ... reportedly by flying a drone over the event before climbing a roof and opening fire.
The would-be assassin filmed aerial footage with the drone shortly before Trump took the stage ... law enforcement sources told the Wall Street Journal.
Crooks flew the drone along a pre-planned route ... suggesting Trump's shooter did a good amount of research at the site of the Butler, PA rally.
The drone was later found in Crooks' car ... and the Secret Service reportedly did NOT use drones to keep watch over the event.
Crooks flying a drone and recording the rally adds another layer to the litany of security gaffes leading to Trump being shot onstage in an assassination attempt that killed a man in the crowd.
Remember ... the Secret Service now says its counter snipers spotted Crooks on the roof where he took his shots, noticing him 20 minutes before he started firing on Trump.
He was first identified as a person of interest about an hour before the shooting, and then seen with his rangefinder, 22 minutes before he was first eyeballed on the rooftop.
Footage also shows Crooks lurking around the building before making his ascent, and there's video of attendees pointing Crooks out to law enforcement before the assassination attempt.
Add Crooks' drone to the list of questions Congress will have for the Secret Service.