Donald Trump assassin Thomas Matthew Crooks was spotted wandering through merch stalls less than two hours before he wounded the ex-president.

Crooks grazed Trump in the right ear as he spoke at a rally in Butler, Pennsylvania, on July 13, and killed supporter Corey Comperatore in the crowd.

Trump was shot at 6:11 pm by the first of eight bullets Crooks fired from the roof of a nearby building until he was gunned down by a Secret Service counter-sniper. 

Joe Tomko, owner of Iron Clad USA, inadvertently filmed Crooks strolling past his booth outside the rally at 4:26 pm - the earliest known recording of him at the rally.

This was exactly the same time a local police sniper texted his colleagues to say he had seen a man, believed to be Crooks, sitting at a picnic table nearby.

Tomko told DailyMail.com that he had no idea he filmed Crooks, 20, until he looked at the footage a week after the shooting on July 20.

'I was have breakfast with my parents and my father asked me if I ever looked through all my footage from that day,' he said.

'So I watched it all late in the evening and I just about jumped out of my skin [when I saw him], I was in such disbelief.

Crooks' movements were meticulously tracked since the shooting to determine what went wrong, and Tomko said he gave the video to the FBI for analysis. 

Crooks was filmed as the camera panned from the stall, selling political-themed hats and T-shirts, around the fairground to show how busy it was.

The video showed Crooks in the same gray T-shirt, made by the Demolition Ranch firearms influencer's page, and white shorts as he had on when he was killed.