US Secret Service Director Kimberly Cheatle has resigned, US media reported Tuesday, a day after she acknowledged that the agency failed in its mission to prevent the assassination attempt against Donald Trump.

Cheatle was facing bipartisan calls to step down after a 20-year-old gunman wounded the Republican presidential candidate at a July 13 campaign rally in Pennsylvania.

"The Secret Service's solemn mission is to protect our nation's leaders," Cheatle said during a contentious hearing of the House Committee on Oversight and Accountability on Monday.

"On July 13, we failed," she said. "As director of the United States Secret Service, I take full responsibility for any security lapse."

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."

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

Both Republicans and Democrats called on Cheatle to resign and she drew the ire of lawmakers from both parties by refusing to provide specific details about the attack, citing the existence of multiple active investigations. 

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

The Secret Service was alerted "two to five times" ahead of the shooting about a "suspicious individual" at the rally, she said, but he was not immediately classified as a "threat."

"There were teams that were sent to identify and interview that individual," she said, but they were unable to locate him before he opened fire. 

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 President Joe Biden in 2022.