Two witnesses at Donald Trump's election rally in Pennsylvania, where he was attacked, claimed to have seen the shooter, with one recalling how the gunman moved from roof to roof, apparently scouting for a perfect perch to shoot at the former US president.

The FBI on Sunday identified the gunman as Thomas Matthew Crooks, 20, from Bethel Park, Pennsylvania. Crooks, armed with an AR-style rifle, was killed by Secret Service personnel. He fired multiple shots at the stage from an “elevated position outside of the rally venue,” the agency said.

The gunman was able to get close enough to shoot and injure Trump is seen as a huge failure of the security agencies tasked with providing cover for him ahead of the election.

The gunman was on a rooftop approximately "200 to 250 yards" from where the former US president was addressing his supporters, US media reported.

According to an attendee named Ben Macer, he was up along the fence line and "saw the guy move from roof to roof", CBS News reported.

Macer further said he told an officer that the shooter was on the roof, approximately "200 to 250 yards" away from where Trump was addressing his supporters.

"When I turned around to go back to where I was, it was when the gunshots started, and then it was just chaos, and we all came running away, and that was that," he said.

Butler resident Ryan Knight, another witness along the fence line, also said he saw the suspected shooter atop the American Glass Research building, the report said.

"I walked over about 20 minutes before the shooting happened to stand along the fence line where Trump was. As I was doing that, I was right beside the AGR building, which had the shooter on it," Knight said.

"When I was sitting there, a guy said, 'Oh God, he had a gun.' When I looked up, there was a guy on top of the building with an M16 with a blanket, pointing at the president. He starts shooting. Four to five shots rang out. I throw the guy I'm with to the ground. I jump to the ground, I look up, and I see his head get split up from the shot from the Secret Service," he said.

The Secret Service has confirmed that one attendee was killed and two others were critically injured in the shooting.

"All the cops started flying over, pushing us out, they took my name, witness report, [and] cell phone number to go over what happened because there [weren't many] eyewitnesses," Knight said.

"My main thought is why wasn't the Secret Service on top of AGR? That's a very big vantage point to shoot at the president. How did that get missed?" Knight asked.

Macer said he did not get a clear look at the alleged shooter, but reiterated that he saw him move from building to building.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.