The investigation into the shooting of former President Donald Trump at a rally in Pennslyvania has revealed that Secret Service snipers saw the shooter, Thomas Matthew Crooks, on the roof of a nearby building 20 minutes before he attempted to assassinate Trump.

Crooks was first spotted at 5:52 p.m. local time, and the shooting started at 6:12 p.m., law enforcement officials including the FBI said.

Officials also found that Crooks was first identified as a person of interest more than an hour before the shooting started, at 5:10 p.m.

At 5:30 p.m., 40 minutes before the shooting, Crooks was seen with a rangefinder, but no action was taken to intercept him.

Quickly taken out

Only 26 seconds after the shooting started, snipers shot and killed Crooks, the investigation said.

The Secret Service has come under fire for failures in security that allowed the shooting, which killed one rallygoer, seriously wounded two others, and grazed Trump's ear.

Director Kimberly Cheatle has made various excuses for the failures, including the slope of the roof and not wanting to shoot someone unless they knew he planned to harm Trump.

Meanwhile, it has been discovered that Crooks appeared to have planned the assassination attempt for weeks beforehand.

Investigators now know he bought ammunition at Walmart two days after the rally was announced.

He went to the rally site the morning of the rally before returning a little after 5 and trying to enter the security perimeter.

No motive

Besides some vague indications that Crooks didn't like politicians, investigators have found no indication of motive so far. His parents, both licensed counselors, do not appear to have strong political ideologies either.

The FBI has conducted 200 interviews so far, officials told lawmakers briefed on the situation.

They have determined that Crooks climbed onto an air conditioning unit and then the roof of a building, which he used to gain access to the American Glass Research building from which he took the shots at Trump.

Police and snipers were inside the building while Crooks was on top of it. They were looking for him after he was seen pacing outside the metal detectors, but he faded into the crowd and could not be found.

Crooks also had a remote detonator in his pocket that could have set off explosive devices found in his car parked near the rally site.