A day after former US president Donald Trump narrowly survived an assassination attempt at a rally, there was much commentary as to whether the Secret Service had provided adequate security.

The sniper, identified as Thomas Crooks, 20, fired rounds from a semiautomatic AR-15-style weapon as he was perched on a rooftop in Butler, Pennsylvania. Trump said one bullet pierced his right ear. Another shot killed a spectator at the rally on Saturday.

Crooks, a resident of Bethel Park, a Pittsburgh suburb, was killed by Secret Service snipers who were positioned on another rooftop, but not before he managed to fire several rounds.

A local police officer climbed to the roof and encountered Crooks, who pointed his rifle at the officer, The Associated Press reported. The officer retreated down a ladder, and Crooks quickly took a shot toward Trump. That is when the Secret Service counter-snipers shot him, said officials who spoke to the AP on condition of anonymity.

The officials also said that bomb-making materials were found inside Crooks' vehicle and at his home.

Questions arose as to how Crooks was able to go unnoticed by security as he scaled a building 150 yards (137 meters) away. A couple of eyewitnesses told reporters that they saw Crooks atop the building and yelled repeatedly for law enforcement to respond.

The building where Crooks was is home to AGR International, a manufacturing plant just north of the Butler Farm Show grounds where Trump's rally was held.

"We're not looking at any skyscrapers here," said Robert E. McDonald, a lecturer at the University of New Haven who spent 20 years in the Secret Service," to The New York Times. "They should be able to see that."

Former Secret Service director Julia Pierson told USA Today: "I think 1,000 yards is the sniper capability that we have a concern about for the president. So anything that's within that range, that is a professional, makeable shot," she said.

"When you think about it, it's a football field and a half … and that is a makeable shot by an individual. And obviously an inch would have made a difference in this case and Trump wouldn't be with us," Pierson said.

Asked at a news conference whether law enforcement did not know the shooter was on the roof until he began firing, Kevin Rojek, the agent in charge of the FBI's Pittsburgh field office, responded that "that is our assessment at this time".

"It is surprising" that the gunman was able to open fire on the stage before the Secret Service killed him, Rojek added.

The FBI has taken the lead role in the investigation, the bureau said on X on Saturday. On Sunday, the FBI said the shooter likely acted alone.

A former Secret Service agent told Business Insider that countersnipers should have provided "360-degree coverage".

"I don't know how many they had, but they usually always look for 360-degree coverage," former agent Anthony Cangelosi said.

Bill Pickle, a former deputy assistant Secret Service director, told The Wall Street Journal that how the agency communicated with local law enforcement and used technology such as drones will be investigated.

"The reality is there's just no excuse for the Secret Service to be unable to provide sufficient resources to cover an open rooftop 100 yards away from the site," he said. "And there's no way he should've got those shots off."

A Secret Service spokesman said on X on Sunday that reports that a member of Trump's team had requested additional security was "untrue".

"There's an untrue assertion that a member of the former President's team requested additional security resources & that those were rebuffed. This is absolutely false. In fact, we added protective resources & technology & capabilities as part of the increased campaign travel tempo," spokesman Anthony Guglielmi wrote.

Authorities identified the rally attendee who was shot and killed as Corey Comperatore, 50, a former volunteer fire chief, of Sarver, Pennsylvania. Governor Josh Shapiro told reporters that Comperatore was fatally shot when he dived on top of his family to protect them from the hail of bullets.

Two other attendees were critically wounded.

Trump is due to receive his party's formal nomination at the Republican National Convention, which kicks off in Milwaukee, Wisconsin, on Monday. He spent Saturday night at his golf club residence in Bedminster, New Jersey.

"I was going to delay my trip to Wisconsin, and The Republican National Convention, by two days, but have just decided that I cannot allow a 'shooter,' or potential assassin, to force change to scheduling, or anything else. Therefore, I will be leaving for Milwaukee, as scheduled," Trump wrote on his Truth Social site on Sunday. Trump arrived in Milwaukee around 7 pm EDT Sunday.

"I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin," he wrote of the shooting. "Much bleeding took place."

In a statement Sunday, former first lady Melania Trump said that when she saw her husband wounded, "I realized my life, and Barron's life, were on the brink of devastating change," referring to their son.

Crooks was a registered Republican, according to state voter records, and had made a $15 donation to a Democratic political action committee at the age of 17 on Jan 20, 2021, the day President Joe Biden was sworn in.

In the third time that Biden addressed the shooting, the president said from the Oval Office on Sunday night that it was time to "lower the temperature" in American politics.

"All of us now face a time of testing as the election approaches," he said.

In brief remarks earlier Sunday, Biden called the shooting "contrary to everything we stand for us as a nation, everything".

"We cannot, we must not, go down this road in America," he said.

Biden, who is running against Trump in a rematch of 2020, said the two men had a "short but good" conversation Saturday night.

He said Trump has been provided "every resource" for his security. The president added that he is directing a review of security at the rally and for the Republican convention.

Whether Saturday's shooting will eventually lower the political temperature in the US ahead of the Nov 5 presidential election remains to be seen.

Many Republicans quickly blamed the violence on Biden and his allies, arguing that sustained attacks on Trump as a threat to democracy have created a toxic environment. They pointed in particular to a comment Biden made to donors on July 8, saying "it's time to put Trump in the bullseye".

Homeland Security Secretary Alejandro Mayorkas, whose department oversees the Secret Service, said officials were engaged with the Biden and Trump campaigns and "taking every possible measure to ensure their safety and security".

House Oversight Committee Chairman James Comer said in a statement on Saturday that he has "already contacted the Secret Service for a briefing and am also calling on Secret Service Director Kimberly Cheatle to appear for a hearing."

Saturday's shooting has increased calls for independent presidential candidate Robert F. Kennedy Jr. to receive Secret Service protection. Kennedy, whose father, Senator Robert F. Kennedy, was assassinated in 1968, and uncle, President John F. Kennedy, was assassinated in 1963, has said he has been paying for his own private security at a high cost.

Colorado Governor Jared Polis, a Democrat, wrote on X on Saturday that he encourages Biden to "immediately provide" Secret Service protection for Kennedy.

Representative Ritchie Torres, a New York Democrat, wrote on X on Sunday: "Congressman Mike Lawler (a New York Republican) and I are putting aside partisan differences to make a bipartisan push for enhanced Secret Service Protection for President Joe Biden, former President Donald Trump, and presidential candidate Robert F Kennedy Jr. We plan to introduce bipartisan legislation in the wake of the attempted assassination."

Agencies contributed to this story.