WASHINGTON -- US Secret Service Director Kimberly Cheatle resigned on Tuesday after widespread outrage over the agency's failure to prevent the recent assassination attempt on former president Donald Trump at a campaign rally in Pennsylvania earlier this month.

Cheatle's resignation came one day after she was grilled at a congressional hearing by lawmakers, who said she failed to answer basic questions and demanded that she step down immediately.

At the hearing held by the House Committee on Oversight and Accountability, Cheatle acknowledged that it was "the most significant operational failure at the Secret Service in decades".

The committee issued a statement following the hearing. "Republican and Democrat members pressed US Secret Service Director Kimberly Cheatle for answers on the egregious security lapses that led to the attempted assassination of (former) President Trump, the murder of an innocent victim, and harm to others in the crowd at the campaign rally in Butler, Pennsylvania," it said.

"Director Cheatle could not or refused to answer basic questions from Committee members, leading Republican and Democrat members to call for Director Cheatle's immediate resignation," the committee said.

At the hearing, Cheatle frequently stated that it's only 9 days after the shooting, and she was still waiting for reports of the ongoing investigations, emphasizing her effort to ensure the information she provided was accurate.

When asked why the Secret Service failed to secure a rooftop in proximity to the rally site and with a clear view of Trump's podium, Cheatle suggested that the Secret Service had intentionally left the rooftop unattended, as the agency preferred "sterile" rooftops, meaning empty ones.

She explained that the warehouse rooftop was meant to have "overwatch", with law enforcement officers monitoring it from a higher vantage point. "There was a plan in place to provide overwatch and we are still looking into responsibilities and who was going to provide overwatch."

When asked whether the Trump detail requested additional resources before the event, Cheatle said that "for the event on July 13th, the assets that were requested for that day, were given".

A recorded footage clearly showed the gunman, who was getting into position to assassinate Trump, caught the attention of some people around. When asked why the rally wasn't paused at that point when people alarmed officers, Cheatle said "we are currently still combing through communications and when communications were passed."

The US Secret Service has been facing scrutiny after a 20-year-old man, identified by the Federal Bureau of Investigation (FBI) as Thomas Matthew Crooks, armed with an AR-style rifle, was able to get close enough to shoot and injure the former president.