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 in Pennsylvania, a monumental failure of one the agency's core duties.
The gunman, who was killed by Secret Service personnel on Saturday (Sunday morning AEST), fired multiple 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 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 man wearing grey camouflage lying motionless on the roof of a manufacturing plant just north of the Butler Farm Show grounds, where Trump's rally was held.
The roof 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 meters is a distance at which US Army recruits must hit a scaled human-sized silhouette to qualify with the M16 assault rifle in basic training. The AR-15, like the shooter at the Trump rally had, is the semi-automatic civilian version of the military M16.
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, 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."
The FBI early on Sunday (Sunday night AEST) identified the shooter as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania.
The Secret Service didn't have anybody 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 counter assault team were at the rally, according to two law enforcement officials. The officials spoke on condition of anonymity because they weren't authorised to discuss details of the investigation.
The heavily armed counter assault team, whose Secret Service code name is "Hawkeye", is 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", uses long-range binoculars and is equipped with sniper rifles to deal with long-range threats.
US Secretary of Homeland Security Alejandro Mayorkas said his department and the Secret Service were working with law enforcement to investigate the shooting.
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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," Mayorkas said.
"We are engaged with President 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.
James Comer, a Kentucky Republican who is the House Oversight Committee chairman, said he contacted the Secret Service for a briefing and called on Director Kimberly Cheatle to appear for a hearing. Comer said his committee will send a formal invitation soon.
"Political violence in all forms is unamerican and unacceptable. There are many questions and Americans demand answers," Comer said in a statement.
US Representative Ritchie Torres, a New York Democrat, called for "security failures" at the rally to be investigated.
"The federal government must constantly learn from security failures in order to avoid repeating them, especially when those failures have implications for the nation," Torres said.
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Wisconsin Governor Tony Evers, a Democrat, posted on X that he and his staff were in contact with security planning coordinators ahead of the Republican National Convention set to begin Monday in Milwaukee.
"We cannot be a country that accepts political violence of any kind — that is not who we are as Americans," Evers said.
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," Garland said in a statement.
"We will not tolerate violence of any kind, and violence like this is an attack on our democracy."