On the heels of an apparent attempt to kill him, former President Donald Trump called Sunday for unity and resilience as shocked leaders across the political divide recoiled from the shooting that left him injured but “fine” and the gunman and a rally-goer dead.

The presumptive Republican presidential nominee said the upper part of his right ear was pierced in the shooting. His aides said he was in “great spirits” and doing well.

“I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin,” he wrote on his social media site. “Much bleeding took place.”

In a subsequent post Sunday, Trump said “it was God alone who prevented the unthinkable from happening."

“In this moment, it is more important than ever that we stand United, and show our True Character as Americans, remaining Strong and Determined, and not allowing Evil to Win,” his post said.

The FBI early Sunday identified the shooter as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania, as it pressed its investigation.

Secret Service agents fatally shot Crooks. The gunman attacked from an elevated position outside the rally venue at a farm show in Butler, the agency said.

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

The attack was the most serious attempt to kill a president or presidential candidate since Ronald Reagan was shot in 1981. It drew new attention to concerns about political violence in a deeply polarized US less than four months before the presidential election. And it could alter the tenor and security posture at the Republican National Convention, which will begin Monday in Milwaukee.

Organisers said the convention would proceed as planned.

Trump flew to New Jersey after visiting a local Pennsylvania hospital, landing shortly after midnight at Newark Liberty International Airport. Video posted by an aide showed the former president leaving his private jet flanked by US Secret Service agents and heavily armed members of the agency's counter assault team, an unusually visible show of force by his protective detail.

President Joe Biden, who is running against Trump, was briefed on the attack and spoke to Trump several hours after the shooting, the White House said.

“There's no place in America for this type of violence,” the president said. “It's sick. It's sick.”

Biden planned to return to Washington early, cutting short a weekend at his beach home in Rehoboth Beach, Delaware.

Many Republicans quickly blamed the violence on Biden and his allies, arguing that sustained attacks on Trump as a threat to democracy have created a toxic environment. They pointed in particular to a comment Biden made to donors on July 8, saying “it's time to put Trump in the bullseye.”

Officials said members of the 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 center of protection.

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.

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

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 US 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.

Homeland Security Secretary Alejandro Mayorkas, whose department oversees the Secret Service, said officials were engaged with the Biden and Trump campaigns and “taking every possible measure to ensure their safety and security.”