The US Secret Service is investigating how a gunman armed with an AR-style rifle was able to get close enough to shoot and injure former president Donald Trump at a rally on Saturday in Pennsylvania, a monumental failure of one of the agency’s core duties.

The gunman, who was killed by Secret Service personnel, fired several shots at the stage from an “elevated position outside of the rally venue”, the agency said.

An Associated Press analysis of more than a dozen videos and photos taken at the Trump rally, as well as satellite imagery of the site, shows the gunman was able to get astonishingly close to the stage where the former president was speaking.

A video posted to social media and geolocated by the AP shows the body of a man wearing grey camouflage lying motionless on the roof of a manufacturing plant just north of the Butler Farm showgrounds, where Mr Trump’s rally was held.

Two police snipers with guns on tripods
Police snipers return fire after shots were fired while Donald Trump was speaking at a rally (Gene J. Puskar/AP)

The roof was less than 150 metres from where Mr 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 US army recruits must hit a human-sized silhouette to qualify with the M16 assault rifle in basic training.

The AR-style rifle, like the gunman at the rally had, is the semiautomatic civilian version of the military M16.

The FBI on Sunday identified the shooter as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania.

The Secret Service did not have a speaker at a late-night news conference where FBI and Pennsylvania State Police officials briefed reporters on the shooting investigation.

FBI special agent in charge Kevin Rojek said it was “surprising” that the gunman was able to fire at the stage before he was killed.

Members of the Secret Service’s counter-sniper team and counterassault team were at the rally, according to two law enforcement officials.

The heavily armed counter assault team, whose Secret Service code name is Hawkeye, are responsible for eliminating threats so that other agents can shield and take away the person they are protecting.

The counter-sniper team, known by the code name Hercules, use long-range binoculars and are equipped with sniper rifles to deal with long-range threats.

US secretary of homeland security Alejandro Mayorkas said his department and the Secret Service are working with law enforcement to investigate the shooting.

Maintaining the security of presidential candidates and their campaign events is one of the department’s “most vital priorities”, he said.

“We condemn this violence in the strongest possible terms and commend the Secret Service for their swift action today,” Mr Mayorkas said.

“We are engaged with President (Joe) Biden, former president Trump and their campaigns, and are taking every possible measure to ensure their safety and security.”

Calls for an investigation came from all sides.

The FBI said it will lead the investigation into the shooting, working with the Secret Service and local and state law enforcement.

Attorney general Merrick Garland said the justice department “will bring every available resource to bear to this investigation”.

“My heart is with the former president, those injured, and the family of the spectator killed in this horrific attack,” Mr Garland said in a statement.

“We will not tolerate violence of any kind, and violence like this is an attack on our democracy.”