Who was the gunman who tried to assassinate Donald Trump?

The FBI has named Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania, as the man they believe shot of former US President Donald Trump. The town is about an hour's drive away from the rally that Trump was addressing.

Secret Service agents then shot Crooks dead as he lay on the roof of a building just outside the rally venue at a farm show in Butler, Pennsylvania, the agency said.

One attendee was also killed and two spectators were critically injured, authorities said. All were identified as men.

Media reports in the US say Crooks was registered as a Republican in Allegheny County, Pennsylvania, according to county voter records.

But US federal campaign-finance records also show that Crooks donated about 14 euros to the Progressive Turnout Project, a liberal group that encourages Americans to vote.

According to the USA Today newspaper he doesn't have a criminal record in Pennsylvania.

A video posted to social media and geolocated by the AP shows the body of a person wearing grey camouflage lying motionless on the roof of a building at AGR International Inc., a manufacturing plant just north of the Butler Farm Show grounds where Trump’s rally was held.

Officials said members of the U.S. Secret Service counterassault team killed the shooter. The heavily armed tactical team travels everywhere with the president and major party nominees and is meant to confront any active threats while other agents focus on safeguarding and evacuating the person at the centre of protection.

Law enforcement recovered an AR-style rifle at the scene, according to a third person familiar with the matter who spoke on condition of anonymity to discuss the ongoing investigation.

An AP analysis of more than a dozen videos and photos from the scene of the Trump rally, as well as satellite imagery of the site, shows the shooter was able to get astonishingly close to the stage where the former president was speaking.

The roof where the person lay was less than 150 metres from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. For reference, 150 metres is a distance at which U.S. Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle. The AR-15, like the shooter at the Trump rally had, is the semi-automatic civilian version of the military M-16.

Asked at the press conference whether law enforcement did not know the shooter was on the roof until he began firing, Kevin Rojek, Special Agent in Charge of the FBI’s Pittsburgh Field Office, responded that “that is our assessment at this time.”

“It is surprising” that the gunman was able to open fire on the stage before the Secret Service killed him, he added

The attack was the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981. It drew new attention to concerns about political violence in a deeply polarised U.S. less than four months before the presidential election.