Homeland Security Secretary Alejandro Mayorkas said Monday that a gunman — one of whose volley of shots hit former US president Donald Trump in the ear — never should have reached his rooftop position.

"We are speaking of a failure," Mayorkas told CNN. "We are going to analyze through an independent review how that occurred, why it occurred, and make recommendations and findings to make sure it doesn't happen again."

Mayorkas said that he still has "full confidence" in Secret Service's leadership.

The top of Trump's right ear was torn by a bullet in the shooting at a rally Saturday in Butler, Pennsylvania, in which one of his supporters was killed and two others wounded before Secret Service agents shot dead the 20-year-old suspect, Thomas Crooks.

Trump had turned his head slightly clockwise to look at a chart just before the bullet struck, which likely saved his life.

At the rally, police had received a report of a suspicious man pacing near magnetometers. Witnesses pointed and shouted at an armed man on a nearby roof.

When a Butler Township police officer climbed up to the roof to investigate, the gunman turned and pointed his rifle at him. But the officer did not — or could not — fire a single shot.

Investigators are trying to determine how an armed man with no military background managed to reach high ground and get the jump on Secret Service agents.

At least a dozen police officers and sheriff's deputies were assisting the Secret Service and Pennsylvania State Police with rally security.

The Butler Township officer was hoisted by another officer so he could grab the edge of the roof, local officials said.

The officer dropped back down to safety when the gunman turned and pointed his rifle at him, according to Butler County Sheriff Michael Slupe.

Slupe, who did not witness the encounter, said the officer could not have wielded his own gun under the circumstances.

"I think all law enforcement on site did everything that they could, especially the local law enforcement," Slupe told The Associated Press on Monday. "I hope they're not made a scapegoat, because they did their job to the best of their abilities."

Butler Township Manager Tom Knights said the officer lost his grip and was not retreating when he fell 8 feet to the ground.

"He was literally dangling from the edge of a building and took the defensive position he needed to at that time. He couldn't hold himself up," Knights said.

The officer, who was not identified, severely injured an ankle in the fall, Knights said.

A former fire chief, 50-year-old Corey Comperatore, was killed in the shooting. A GoFundMe campaign for the man's family has raised more than $1 million.

A law enforcement official told AP on condition of anonymity that investigators believe Crooks purchased 50 rounds of ammunition on the day of the shooting.

Authorities said they believe the gunman's AR-style rifle was purchased by his father.

Kevin Rojek, FBI special agent in charge in Pittsburgh, said investigators do not yet know if Crooks took the gun without his father's permission.

The FBI believes Crooks, who had bomb-making materials in the car he drove to the rally, acted alone.

The roof where Crooks lay was less than 150 meters (164 yards) from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. That is a distance at which US Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle.

The FBI said on Monday it had gained access to Crooks' phone as it searched for a motive.

"FBI technical specialists successfully gained access to Thomas Matthew Crooks' phone, and they continue to analyze his electronic devices," the bureau said in a statement. "The search of the subject's residence and vehicle are complete."

The AR-style 556 rifle used by the shooter was legally purchased by his father and officials said there was no indication that the suspect suffered from any mental health issues.

They added that a preliminary review of Crooks' calls and texts did not shed any light on his motive.

In the aftermath of the shooting, President Joe Biden directed the Secret Service to protect independent presidential Robert F. Kennedy Jr., Mayorkas said Monday.

Kennedy's campaign has been urging the president to provide him with the protection for months and has sent multiple requests.

Kennedy's uncle, President John F. Kennedy, and his father, Senator Robert F. Kennedy, were both assassinated less than five years apart in the 1960s.

On Monday, Trump urged the government to provide the Secret Service detail to Kennedy, who had repeatedly been denied the protection.

"Given the history of the Kennedy Family, this is the obvious right thing to do!" Trump wrote on Truth Social.

"Thank you, President Biden, for extending me Secret Service protection," Kennedy said in a statement.

In October, a man was arrested after trespassing twice in one day at Kennedy's Los Angeles home, and a month earlier, an armed man accused of impersonating a federal officer was taken into custody outside a Kennedy campaign event.

Kennedy also thanked his private security firm, Gavin de Becker & Associates, "for keeping me safe for the past 15 months of my presidential campaign".

Though Kennedy is a long shot to win Electoral College votes, much less the presidency, his campaign events have drawn large crowds people interested in his message.

Amid calls for a toning down of political rhetoric in the US, Biden said in an interview Monday that it was "a mistake" to say that he wanted to put Trump back in "a bullseye", but also stood by his depictions that the former president is "a threat to democracy".

"How do you talk about the threat to democracy, which is real, when a president says things like he says?" Biden said to NBC's Lester Holt in an excerpt from the interview.

"Do you just not say anything because it may incite somebody?" he said.

"He talks about there'll be a blood bath if he loses," Biden said. Trump has maintained that he was referring to the economy.

Just last week, Biden faced intense scrutiny and calls for him to step aside as the Democratic candidate from within his own party. The pressure began following Biden's listless performance in a debate against Trump on June 27.

But Biden has dismissed the uproar and has vociferously maintained that he would continue as the candidate. Saturday's shooting has refocused media coverage.

Ai Heping in New York and agencies contributed to this story.