Secret Service Director Kimberly Cheatle said Monday that her agency failed in its mission to protect former President Donald Trump during a highly contentious congressional hearing with lawmakers of both major political parties demanding she resign over security failures that allowed a gunman to scale a roof and open fire at a campaign rally.

Cheatle was berated for hours by Republicans and Democrats, repeatedly angering lawmakers by evading questions about the investigation in her first congressional hearing over the July 13 assassination attempt She called the attempt on Trump’s life the Secret Service’s “most significant operational failure” in decades.

“The Secret Service’s solemn mission is to protect our nation’s leaders. On July 13th, we failed,” Cheatle said.

Cheatle acknowledged that the Secret Service was told about a suspicious person two to five times before the shooting. She also revealed that the roof from which Thomas Matthew Crooks opened fire had been identified as a potential vulnerability days before the rally. Cheatle said she apologized to Trump in a phone call after the shooting.

Yet Cheatle remained defiant that she was the “right person” to lead the Secret Service, even as she said she takes full responsibility for any security lapses at the Butler, Pennsylvania, rally.

When Republican Rep. Nancy Mace suggested Cheatle begin drafting her resignation letter from the hearing room, Cheatle responded, “No, thank you."

Democrats and Republicans were united in their exasperation as Cheatle said she didn't know or couldn't answer numerous questions more than a week after the shooting, which left one spectator dead.

"It has been 10 days since an assassination attempt on a former president of the United States. Regardless of party, there need to be answers,” said Alexandria Ocasio-Cortez, D-New York.

Lawmakers peppered Cheatle with questions about how the gunman could get so close to the Republican presidential nominee when he was supposed to be carefully guarded and about why Trump was allowed to take the stage after local law enforcement had identified Thomas Matthew Crooks as suspicious.

Cheatle acknowledged that Crooks had been seen by local law enforcement before the shooting with a rangefinder, a small device resembling binoculars that hunters use to measure distance from a target. She said the Secret Service would never have taken Trump onto the stage if it had known there was an “actual threat." But while Crooks had been reported as suspicious, he wasn't identified as a threat until seconds before the shooting, she said.

“An individual with a backpack is not a threat,” Cheatle said. “An individual with a range finder is not a threat."

Asked about why there were no agents on the roof where the shooter was located or if the Secret Service used drones to monitor the area, Cheatle said she is still waiting for the investigation to play out, prompting groans and outbursts from members on the committee.

“Director Cheatle, because Donald Trump is alive, and thank God he is, you look incompetent," said Rep. Mike Turner, R-Ohio. “If he were killed you would look culpable."

Rep. Ro Khanna, D-Calif., noted that the Secret Service director who presided over the agency when there was an attempted assassination of President Ronald Reagan later stepped down.

“The one thing we have to have in this country are agencies that transcend politics and have the confidence of independents, Democrats, Republicans, progressives and conservatives,” Khanna said, adding that the Secret Service was no longer one of those agencies.

Trump was wounded in the ear, a former Pennsylvania fire chief was killed and two other attendees were injured after Crooks climbed atop the roof of a nearby building and opened fire with an AR-style rifle shortly after Trump started speaking at the July 13 rally in Butler, Pennsylvania.

The Secret Service has acknowledged it denied some requests by Trump's campaign for increased security at his events in the years before the assassination attempt. But Cheatle said that there were “no assets denied" for the rally.

Homeland Security Secretary Alejandro Mayorkas has called what happened a “failure” while several lawmakers have called on Cheatle to resign or for President Joe Biden to fire her. The Secret Service has said Cheatle does not intend to step down. So far, she retains the support of Biden, a Democrat, and 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 meters (157 yards) of the stage. He then set up his 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 Trump was the most serious attempt to assassinate a president or presidential candidate since 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 have not found any ideological bent that could help explain his actions. Investigators who searched his phone found photos of Trump, Biden and other senior government officials and found that he had looked up the dates for the Democratic National Conventional as well as Trump’s appearances. He also searched for information about major depressive order.