The Secret Service is facing questions after former US President Donald Trump was shot at during a rally in Pennsylvania.
Trump, who is now "doing well", says he was shot in the ear as he stood in front of crowds at a fairground in Butler.
One bystander was killed in the shooting and two others were critically injured, according to a Secret Service spokesman.
The Director of the Secret Service, Kimberly Cheatle, has been summoned to testify before the US House of Representatives on 22 July by the Oversight Committee - the main investigative board of the US House of Representatives.
At a news conference on Sunday, FBI special agent Kevin Rojek said it was "surprising" that the shooter, who has been named as 20-year-old Thomas Matthew Crooks, was able to open fire before the Secret Service killed him.
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An investigation into the attempted assassination, which is already under way, involves the FBI, the Secret Service and the Department of Homeland Security.
"Americans demand answers about the assassination attempt of President Trump," the Oversight Committee said in a statement on social media.
Trump was quickly bundled off stage and into a waiting vehicle after shots were fired just a few minutes into his speech at 18:11 local time on Saturday.
Blood could be seen near the former president's ear as he raised his fist to the crowd.
In a post to his Truth Social network, Trump said a bullet 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," Trump wrote.
"Much bleeding took place, so I realised then what was happening."
The FBI said it is treating the incident as an assassination attempt on Trump and it is an "active and ongoing investigation".
The suspect was shot dead at the scene by a US Secret Service sniper, said the agency's spokesperson, Anthony Guglielmi.
He added that the one bystander killed in the shooting, and the two others critically injured, were all male. Their identities have not been released.
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Law enforcement sources told CBS News that Crooks had been armed with "an AR-style rifle" and had fired from a building a few hundred metres from the venue.
Special Agent Kevin Rojek said Crooks had not been carrying ID and that investigators used DNA to identify him.
They have yet to identify a motive for the assassination attempt, he added.
State voter records show that Crooks was a registered Republican, according to US media reports.
He is also reported to have donated $15 to a liberal campaign group in 2021.
A senior adviser to Donald Trump's campaign said there are questions about how prepared the Secret Service was.
Speaking to the BBC World Service, Stephen Moore called it a "scary day".
"Certainly Trump needs more protection - there’s a lot of inquiry now about whether the Secret Service was totally prepared," Mr Moore said.
However, Mr Guglielmi says there is an "untrue assertion" circulating that someone on Trump's security team had requested extra security "resources" and that request was "rebuffed".
"This is absolutely false. In fact, we added protective resources and technology and capabilities as part of the increased campaign travel tempo," Mr Guglielmi said.
Trump had just started addressing his supporters in Butler, Pennsylvania - a crucial swing state in November's election - when the shots started.
Multiple bangs rang out as Trump spoke about his successor, President Joe Biden, and his administration.
Several supporters holding placards and standing behind Trump ducked as the shots were heard.
Bystanders who spoke to the BBC suggested the gunshots may have come from a one-storey building to the right of the stage where Trump was speaking.
One witness - Greg - told the BBC that he had spotted a suspicious-looking person "bear crawling" on the roof of the building about five minutes after Trump took to the stage. He said he pointed the person out to police.
"He had a rifle, we could clearly see him with 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."
Tim - who was also at the rally - told the BBC that he had heard a "barrage" of shots.
"There was a spray which we initially thought was a fire hose, and then the speaker on the right-hand side started coming down," he said.
"Something must have hit the hydraulic lines [which caused it to fall]. We saw President Trump go to the ground and everyone started dropping to the ground because it was chaos."
Warren and Debbie were at the venue and told the BBC they heard at least four gunshots.
They said they both got on the ground as Secret Service agents came through the crowd, shouting for the attendees to get down. People remained calm, they said.
"We couldn't believe it was happening," Warren said.
Debbie said a little girl beside them was crying that she didn't want to die and saying "how is this happening to us?"
"That broke my heart," Debbie said.
Republican Congressman Ronnie Jackson told the BBC that his nephew was injured in the shooting. He sustained a minor wound to his neck and was treated at the scene, Mr Jackson said in a statement.
Speaking from his home state of Delaware, President Biden deplored the attack, calling it "sick".
"There's no place in America for this kind of violence," he said. "Everybody must condemn it."
The White House later said President Biden had spoken with Trump before returning to Washington DC.
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Trump remains locked in a tight contest with President Biden - the presumptive Democratic nominee - in a re-match of the 2020 election.
Politicians of both parties joined Mr Biden in condemning the apparent attack.
Former President Barack Obama said there "is absolutely no place for political violence in our democracy" and that he was "relieved that former President Trump wasn’t seriously hurt".
Trump's former vice-president Mike Pence said he and his wife were praying for his former ally, adding that he urged "every American to join us".
House Minority Leader Hakeem Jeffries said in a statement: “My thoughts and prayers are with former President Trump. I am thankful for the decisive law enforcement response. America is a democracy. Political violence of any kind is never acceptable.”
UK Prime Minister Sir Keir Starmer led international condemnation of the shooting, saying he was "appalled by the shocking scenes at President Trump's rally".
"Political violence in any form has no place in our societies and my thoughts are with all the victims of this attack," he said in a statement.
Trump is still set to accept his party's nomination for president at the convention in Milwaukee on Monday, his campaign managers said. Some had speculated that he had been set to reveal his running mate at the Butler rally.
Some Republicans were quick to blame President Biden over the shooting, accusing him of stoking fears about Trump's potential return to office.
Senator JD Vance, who is thought to be on the shortlist to become Trump’s vice-presidential candidate, said the rhetoric from the Biden campaign had led directly to this incident.
Mike Collins - a Republican congressman - accused the president of “inciting an assassination”.
Meanwhile James Comer, the chair of the powerful House oversight committee, said he would summon the director of the Secret Service before his panel.
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