The director of the Secret Service says the assassination attempt on former president Donald Trump was the agency's "most significant operational failure" in decades.
Director Kimberly Cheatle told a congressional hearing: "On July 13, we failed."
Ms Cheatle says she takes full responsibility for the agency's mis-steps related to the attack at Mr Trump's Pennsylvania rally earlier this month.
Ms Cheatle was testifying before a congressional committee as calls mount for her to resign over security failures at a rally where a 20-year-old gunman attempted to assassinate the Republican former president.
The House Oversight Committee heard Ms Cheatle's first appearance before politicians since the July 13 Pennsylvania rally shooting that left one spectator dead.
Mr Trump was wounded in the ear and two other attendees were injured after Thomas Matthew Crooks climbed atop the roof of a nearby building and opened fire.
Politicians have been expressing anger over how the gunman could get so close to the Republican presidential nominee when he was supposed to be carefully guarded.
The Secret Service has acknowledged it denied some requests by Mr Trump's campaign for increased security at his events in the years before the assassination attempt.
Homeland Security secretary Alejandro Mayorkas has called what happened a "failure" while several politicians have called on Ms Cheatle to resign or for President Joe Biden to fire her.
The Secret Service has said Ms Cheatle does not intend to step down. So far, she retains the support of Mr Biden, a Democrat, and Mr Mayorkas.
Before the shooting, local law enforcement had noticed Crooks pacing around the edges of the rally, peering into the lens of a rangefinder toward the rooftops behind the stage where the president later stood, officials have told The Associated Press.
An image of Crooks was circulated by officers stationed outside the security perimeter.
Witnesses later saw him climbing up the side of a squat manufacturing building that was within 135 metres from the stage. He then set up his AR-style rifle and lay on the rooftop, a detonator in his pocket to set off crude explosive devices that were stashed in his car parked nearby.
The attack on Mr Trump was the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981. It was the latest in a series of security lapses by the agency that has drawn investigations and public scrutiny over the years.
Authorities have been hunting for clues into what motivated Crooks, but so far have not found any ideological bent that could help explain his actions.
Investigators who searched his phone found photos of Mr Trump, Mr Biden and other senior government officials, and also found that he had looked up the dates for the Democratic National Conventional as well as Mr Trump's appearances.
He also searched for information about major depressive order.
