Dramatic new footage from the Donald Trump assassination attempt shows the moment an officer was hoisted onto the roof where shooter Thomas Matthew Crooks had taken up position.

Body cam footage from the July 13 rally in Butler, PA was just released ... and this angle is from the cop who was confronted by Crooks on the roof.

The full video is 6 minutes long and the first 2 minutes do not have sound ... but you see the cop getting brought up to the roof of the American Glass Research Building.

This is the same officer who claims Crooks pointed his AR-15-style rifle at him. After the confrontation, the cop dropped to the ground and radioed in that Crooks was strapped with a "long gun."

In the video, the cop is on the roof for a few seconds before he hops down and runs like a bat out of hell. You do not see Crooks in this part of the footage.

The officer then runs around the side of the building, towards the stage where Trump is speaking ... and while there's no sound, the timestamp indicates this is when Crooks opened fire ... at 6:11 PM.

The cop then goes back to his squad car and gets a bigger gun and some more backup ... and there's chaos as law enforcement gets back on the roof where the shooter is now lying dead.