In short:
Witnesses say they alerted police to a gunman on a roof outside the political rally several minutes before shots were fired towards Donald Trump.
Mr Trump has survived the shooting, which is being investigated as an assassination attempt.
What's next?
More details are expected on the suspected shooter, who was killed in the incident, and possible motives, as the FBI undertakes an investigation.
Several minutes before a series of shots were fired towards Donald Trump at a political rally in the United States, a gunman was spotted on a nearby rooftop, according to witnesses.
Mr Trump was in the middle of a speech when the volley of "pops" were heard.
The former president grabbed at his ear, and blood was seen streaming down his face as he was swarmed by Secret Service agents.
He turned to the crowd and lifted his fists in the air before being escorted off stage.
The incident at the event in Butler County, Pennsylvania, is being investigated as an assassination attempt.
A witness, named Greg, told the BBC he had alerted to police to a man sprawled on top of a building just outside the event.
He was surprised that Mr Trump's speech was still allowed to go ahead.
"I'm thinking to myself 'Why is Trump still speaking, why have they not pulled him off the stage' … the next thing you know, five shots ring out," he said in a video interview.
Mr Trump's campaign say he was "fine".
The suspected gunman and at least one attendee are dead, local prosecutors said.
Greg said he tried to alert the authorities for three to four minutes.
He believes they were unable to see the suspected gunman due to the slope of the roof.
"Why is there not Secret Service on all of these roofs here?" he asked.
"This is not a big place."
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Another witness shared a similar story with CBS news.
He said while he and other attendees were waiting for Mr Trump to take the stage, they noticed two police officers "looking for something or somebody".
"I was looking around myself and seen [sic] a guy on top of one of the building go in-between one of the buildings to the next," he said.
"I went and told the officer he was up there, and when I went back to my spot, people could still see the person from where they were standing."
He tried to tell the officers where to position themselves to see the suspected gunman.
But at that point, "that's when the shots started".
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'No weapons should get in'
Authorities have confirmed the suspected gunman was positioned on a roof on a building outside the event.
Anthony Guglielmi, a spokesman for the Secret Service, said the suspect fired "multiple shots toward the stage" and was on "an elevated position" outside the rally venue.
Images from the scene show a building surrounded by authorities, positioned over Mr Trump's right shoulder from where he was was speaking.
Those who attend Mr Trump's campaign rallies are subject to security screenings and their belongings are searched for weapons.
There are also typically law-enforcement officials stationed on nearby rooftops when possible.
The witness who spoke to the BBC said they could clearly see the man on the roof holding a rifle.
"He had a rifle, we could clearly see a rifle," he said.
"We're pointing at him, 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 did not know what was going on," he said.
He later saw police shoot the gunman.
"They crawled up on the roof, they had their guns pointed at him, made sure he was dead. He was dead, and that was it — it was over."
The attack was the first attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981.
Professor Claire Finkelstein from the University of Pennsylvania Law School, said the assassination attempt had been "very close".
"As we know, the shooter was shooting from the right hand side at an elevated position," she told ABC News.
"No weapons should get into the immediate area, given the the searches that they do and the equipment that they have for searching for weapons.
"But it's very difficult to control an outdoor area, and all of the positions from which a shooter could fire with a long gun."
Shooter identified
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The FBI early Sunday named Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania, as the shooter in the assassination attempt.
The agency said the investigation remains active and ongoing.
The political leanings of Crooks were not immediately clear.
Records show Crooks was registered as a Republican voter in Pennsylvania, but federal campaign finance reports also show he gave $US15 to a progressive political action committee on January 20, 2021, the day President Joe Biden was sworn into office.
Professor Finkelstein said given a rise in violent rhetoric around political events, future outdoor rallies may be called off.
She noted similarities with the John F Kennedy assassination in 1963.
"[He] was famously killed in an open air circ*mstance in some similar way from somebody at an elevated position on a building at a lookout site," Professor Finkelstein said.
"When you are in an open air situation, security is very tenuous at best. And that may mean they are, for now, simply be off the table."
Professor Finkelstein said more about the motives of the shooter was likely to unfold in the coming days and weeks.
But, like with the Kennedy assassination, there may never be a full picture, especially not by the time of the November election.
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