Donald Trump arrived on Sunday in Milwaukee, where he will be formally nominated as the Republican presidential candidate later this week after surviving an assassination attempt that has aggravated an already bitter U.S. political divide.

US President Joe Biden, a Democrat, ordered a review of how a 20-year-old man with an AR-15-style rifle got close enough to shoot at Trump from a rooftop on Saturday. Trump, as a former president, has lifetime protection by the U.S. Secret Service.

Trump, 78, was holding a campaign rally in Butler, Pennsylvania - a key state in the Nov 5 election - when shots rang out, hitting his right ear and leaving his face streaked with blood. His campaign said he was doing well.

"That reality is just setting in," Trump told the Washington Examiner on Sunday. "I rarely look away from the crowd. Had I not done that in that moment, well, we would not be talking today, would we?"

One person in the crowd was killed and two others wounded before Secret Service agents fatally shot the suspect.

Both Trump and Biden on Sunday sought calm and unity.

Trump is due to accept his party's formal nomination at the Republican National Convention with a speech on Thursday. He pumped his fist in the air several times as he descended the stairs from his plane after arriving in Milwaukee.

"This is a chance to bring the whole country, even the whole world, together. The speech will be a lot different, a lot different than it would've been two days ago," Trump told the Washington Examiner.

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Biden delivered a televised address from the Oval Office in the White House on Sunday.

"There is no place in America for this kind of violence, for any violence ever. Period. No exceptions. We can't allow this violence to be normalized," he said. "The political rhetoric in this country has gotten very heated. It's time to cool it down."

Biden and Trump spoke to each other on Saturday night after the shooting. First Lady Jill Biden also spoke with former First Lady Melania Trump on Sunday afternoon, said a White House official.

Trump and Biden are locked in a close election rematch, according to most opinion polls including by Reuters/Ipsos. The shooting on Saturday whipsawed discussion around the presidential campaign, which had been focused on if Biden, 81, should drop out following a halting June 27 debate performance.

The FBI said there were no known threats to the Republican convention - which kicks off on Monday - or anyone attending, while the Secret Service said they do not anticipate any changes to the security plan.

The convention will feature televised speeches by rising Republican stars and Trump's choice for a yet-to-be-announced vice presidential running mate, while highlighting the party's stance on such topics as abortion, immigration and the economy.

Suspect a nursing home aide

The FBI identified Thomas Matthew Crooks of Bethel Park, Pennsylvania, as the suspect and said the shooting was being investigated as an attempted assassination.

FBI officials said on Sunday that the shooter acted alone. The agency said it had yet to identify an ideology linked to the suspect or any indications of mental health issues or found any threatening language on the suspect's social media accounts.

Crooks was a registered Republican, according to state voter records, and donated $15 to a Democratic political action committee when he was 17. At the time of the shooting he was employed as a dietary aide at a nursing home. The Bethel Park Skilled Nursing and Rehabilitation Center said Crooks "performed his job without concern and his background check was clean."

The gun - an AR-style-5.56 calibre rifle - had been legally bought, FBI officials said, adding they believed it had been purchased by the suspect's father. The officials said "a suspicious device" was found in the suspect's vehicle, which was inspected by bomb technicians and rendered safe.

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The Secret Service denied accusations by some Trump supporters that it had rejected a campaign request for more security, saying that it recently "added protective resources and capabilities to the former President's security detail."

Hours after the assassination attempt, the Oversight Committee in the Republican-led U.S. House of Representatives summoned Secret Service Director Kimberly Cheatle to testify at a hearing scheduled for July 22.

The shots on Saturday appeared to come from outside the area secured by the Secret Service, the agency said.

Butler County Sheriff Michael Slupe said Butler Township police officers had responded to a call about a suspicious person, but were unaware he was armed. He said one officer helped hoist another officer up to look on the roof. As the officer pulled himself up he was confronted by the shooter.

"The shooter heard him or saw him, he turns around with his rifle and of course the guy just lets go and he falls to the ground," said Slupe. The shooter opened fire shortly afterwards, he said.

Spectator killed protecting family

The rally attendee killed on Saturday was identified by authorities as Corey Comperatore, 50, of Sarver, Pennsylvania. He died trying to protect his family from the hail of bullets, said Pennsylvania Governor Josh Shapiro.

"Corey was an avid supporter of the former president, and was so excited to be there," Shapiro said, adding, "Political disagreements can never, ever be addressed through violence."

Two people wounded in the shooting were in a stable condition on Sunday. Pennsylvania State identified them as David Dutch, 57, of New Kensington, Pennsylvania and James Copenhaver, 74, of Moon Township, Pennsylvania.

Residents of Bethel Park, where the suspected shooter lived, expressed shock at the news on Sunday.

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"It's a little crazy to think that somebody that did an assassination attempt is that close, but it just kind of shows the political dynamic that we're in right now with the craziness on each side," said resident Wes Morgan, 42, describing Bethel Park as "a pretty blue-collar type of area."

While mass shootings at schools, nightclubs and other public places are common in the United States, the attack was the first shooting of a U.S. president or major party presidential candidate since the 1981 attempted assassination of Republican President Ronald Reagan.

Americans fear rising political violence, Reuters/Ipsos polling shows, with two out of three respondents to a May survey saying they worried violence could follow the election.

After Biden defeated Trump in the 2020 election, Trump supporters stormed the U.S. Capitol on Jan. 6, 2021, in a deadly riot fueled by Trump's false claims that his loss was the result of widespread fraud.