Washington DC - Secret Service Director Kimberly Cheatle acknowledged on Monday that the agency failed in its mission to prevent the assassination attempt on former president Donald Trump.

"The Secret Service's solemn mission is to protect our nation's leaders," Cheatle said during testimony before the House Committee on Oversight and Accountability.

"On July 13, we failed," she said. "As director of the United States Secret Service, I take full responsibility for any security lapse." But she rejected a wave of bipartisan calls for her resignation.

Cheatle said the attack on Trump, who was slightly wounded in his right ear while speaking at a campaign rally, was "the most significant operational failure of the Secret Service in decades."

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"There clearly was a mistake, and we will make every effort to make sure that this never happens again," she said.

The 20-year-old gunman, Thomas Matthew Crooks, opened fire on Trump with an AR-style assault rifle just minutes after the former Republican president and current White House candidate began speaking at the campaign event in Butler, Pennsylvania.

Crooks, who was perched on the roof of a nearby building with a clear sightline of the stage, was shot dead by a Secret Service sniper 26 seconds after firing the first of eight shots. Investigators have concluded that Crooks, who lived in a town about 50 miles from Butler, acted alone and have not been able to identify any strong ideological or political leanings.

Two rally attendees were seriously wounded in the shooting, and a 50-year-old firefighter, Corey Comperatore, of Freeport, Pennsylvania, was shot dead.

Opening the hearing into the assassination bid, Republican committee chairman James Comer said, "This tragedy was preventable" and "It is my firm belief, Director Cheadle, that you should resign."

"The Secret Service's protective mission is to protect US and visiting world leaders and safeguard US elections through protection of candidates and nominees," Comer said.

"The Secret Service has a zero-fail mission, but it failed on July 13 and in the days leading up to the rally," he said, adding the agency "has now become the face of incompetence."

Representative Michael Turner, a Republican lawmaker from Ohio, also called on Cheatle to step down.

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"Not only should you resign, but if you refuse to do so, President [Joe] Biden needs to fire you because his life, Donald Trump's life, and all the other people who you protect are at risk," Turner said.

Cheatle rebuffed the demands she resign. "I think that I am the best person to lead the Secret Service at this time," she said.

Cheatle declined to answer many specific questions from lawmakers about the attack, saying it was still the subject of multiple active investigations.

"I can speak to you in generalities," she said, drawing the ire of both Republican and Democratic lawmakers on the committee.

Trump's former physician said over the weekend that the Republican candidate sustained a two-centimeter (almost one-inch) gunshot wound on his right ear that is starting to heal.

The memo from ex-White House physician Ronny Jackson, now a hardline right-wing lawmaker from Texas, is the first detailed account of the injury Trump sustained.

"The bullet passed, coming less than a quarter of an inch from entering his head, and struck the top of his right ear," wrote Jackson, who said he flew to see Trump in New Jersey late in the evening of the rally and had been treating him since.

Cheatle served as a Secret Service agent for 27 years before leaving in 2021 to become the head of security in North America for PepsiCo.

She was named to head the agency by Biden in 2022.