The FBI has said it is "too early" to determine if the gunman who shot Donald Trump in the ear, killed an audience member, and critically injured two others at a campaign rally in Pennsylvania acted alone.
The former US president was shot in the ear mid-speech at an event in Butler about 6.10pm on Saturday (8.10am on Sunday AEST).
He was whisked off stage bloodied and shuffled into a security car by Secret Service agents in what law enforcement is treating as an attempted assassination.
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The Secret Service said the shooter fired from "an elevated position outside the rally venue".
As multiple shots were fired from the stage one bullet fatally struck an attendee in the head, and two other attendees were critically wounded.
Agents "neutralised" the shooter.
The FBI this afternoon named Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania, as the subject involved in the assassination attempt.
The agency said the investigation remains active and ongoing.
The agency repeated that Trump is "safe" and is being evaluated.
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An AR-style rifle was recovered from the scene, according to an official.
Trump later took his platform Truth Social to confirm a bullet had pierced the upper part of his right ear.
"I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin." the former president wrote.
"Much bleeding took place, so I realised then what was happening."
Vision shows the former president cowering to the ground before getting to his feet.
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Screams were heard in the crowd of several thousand people and bangs continued as Secret Service agents tended to him on stage.
Trump appeared bloody with his right fist raised in the air before he was shuffled into a security car.
The shooting is being investigated as an attempted assassination of the former president and presumptive Republican nominee, law enforcement officials say.
No motive yet
Speaking at a press conference hours after the incident, special agent in charge of the FBI Pittsburgh field office Kevin Rojek appealed for public information about the deadly shooting.
"The FBI has deployed a number of our resources, including investigative agents, our evidence response team, bomb technicians, and we have additional resources coming from other field offices," he said.
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"We have intelligence analysts as well, working from our field office in Pittsburgh, working feverishly to attempt to identify the individual who did this.
"Anyone who was on scene who saw anything, who identified any information, please report that to the FBI."
Deputy Commissioner of Operations, Lieutenant Colonel George Bivens, would not rule confirm whether the gunman, who killed one audience member and critically injured two others, acted alone.
"I think it's too early to say that. We have one shooter tentatively identified, but we're not stopping there," he told reporters.
"We are absolutely not taking for granted that this was a lone wolf attack and so we would be looking at additional information that could point us to anyone else that may have had a hand in this.
"We're following up on a lot of information."
At this stage, Bivens said authorities are yet to identify the initial shooter and the motive behind the attack.
Bivens said several "suspicious packages" hindered attempts by law enforcement to take down the active shooter when a reporter asked why the suspect wasn't apprehended sooner.
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"I am aware that law enforcement had responded to a number of reports of suspicious activity," he said.
"Their specific response will be part of the investigation after action. But law enforcement was responding to check on several suspicious occurrences."
Authorities said they were not prepared to release the identities of the civilians killed or injured, other than the former president.
Authorities did confirm all three victims were adult and male.
FBI surprised by rapid rounds
Asked if law enforcement did not know the shooter was on the roof until he began firing, Rojek responded: "that is our assessment at this time."
"It is surprising" that the shooter was able to get off as many rounds as were fired, Rojek said.
He added that "all the details of that will come out later investigation."
The Secret Service is investigating how the gunman was able to get close enough to shoot Trump, a monumental failure of one the agency's core duties.
A witness told the BBC that roughly five to seven minutes after Trump started speaking his group noticed a man bear crawling on the building near them.
"We could clearly see him with a rifle. Absolutely. We're pointing at him," the witness said.
"The police are down there running around on the ground. We're like, 'Hey, man, there's a guy on the roof with a rifle, and the police were like, 'Huh, what?' You know, like, they didn't know what was going on.
"You know, we're like, 'hey, right here on the roof', we can see him from right here."
Event to go ahead
Despite the deadly shooting, the Republican National Committee Milwaukee will go ahead as planned on Monday.
RNC says Trump is looking forward to joining the committee.
"President Trump looks forward to joining you all in Milwaukee as we proceed with our convention to nominate him to serve as the 47th President of the United States," said Trump campaign advisers Susie Wiles and Chris LaCivita and Republican National Committee Chair Michael Whatley and co-chair Lara Trump in a statement.
A spokesman for Trump, Steven Cheung, had earlier released a statement saying Trump was "fine".
"President Trump thanks law enforcement and first responders for their quick action during this heinous act," the statement reads.
"He is fine and is being checked out at a local medical facility. More details will follow."
As Trump was talking, a popping sound was heard. About six or seven shots rang out.
Trump said, "Oh," and put his right hand up to his right ear as people in the stands behind him appeared to be shocked. Two more pops could be quickly heard and he crouched down.
Someone could be heard saying near the microphone at Trump's lectern, "Get down, get down, get down, get down!" as agents tackled the former president.
They piled atop him to shield him with their bodies, as is their training protocol, as other agents took up positions on stage to search for the threat.
Police began vacating the fairgrounds shortly after Trump left the stage in what local officers described as a crime scene.
Full coverage of Donald Trump's shooting
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President Joe Biden addressed the incident in a press conference, describing it as "sick".
"There's no place in America for this kind of violence," he said.
"It's one of the reasons why we have to unite this country.
"We cannot allow for this to be happening. We cannot be like this. We cannot condone this."
Biden said he was yet to speak with the Trump but said he understands he's "doing well".
The president fell short of deeming the shooting an assassination "until we have all the facts".
"I have an opinion, but I don't have any facts," he said.
Trump's eldest son, Donald Trump Jr, posted a photo on X of Trump, his fist raised and his face bloody in front of an American flag, with the words: "He'll never stop fighting to Save America."
Prime Minister Anthony Albanese issued a statement this morning describing the incident as "confronting".
"The incident at former President Trump's campaign event in Pennsylvania today is concerning and confronting," Albanese said.
"There is no place for violence in the democratic process.
"I am relieved to hear reports that former President Trump is now safe."
With Associated Press, CNN
