Secret Service Director Kimberly Cheatle said Monday that her agency failed in its mission to protect former President Donald Trump, as lawmakers of both major political parties demanded during a highly contentious congressional hearing that 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 during the first hearing over the July 13 assassination attempt. Cheatle called the attempt on Trump's life the Secret Service's "most significant operational failure" in decades and vowed to "move heaven and earth" to get to the bottom of what went wrong and make sure there's no repeat of it."The Secret Service's solemn mission is to protect our nation's leaders. On July 13th, we failed," she told lawmakers on the House Oversight and Accountability Committee.Cheatle acknowledged that the Secret Service was told about a suspicious person two to five times before the shooting at the Butler, Pennsylvania, rally. 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 assassination attempt.Yet Cheatle remained defiant that she was the "right person" to lead the Secret Service, even as she said she takes full responsibility any security lapses at the event. When Republican Rep. Nancy Mace suggested Cheatle begin drafting her resignation letter from the hearing room, Cheatle responded, "No, thank you."In a rare moment of unity for the often divided committee, the Republican chairman, Rep. James Comer, and its top Democrat, Rep. Jamie Raskin, issued a letter calling on Cheatle to step down.Video below: Secret Service Director Kimberly Cheatle calls assassination attempt a 'significant failure'The White House didn't immediately comment on whether President Joe Biden still has confidence in Cheatle after her testimony.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 that left one spectator dead. At one point, Mace used profanity as she accused Cheatle of lying and dodging questions, prompting calls for lawmakers to show "decorum."Lawmakers pressed Cheatle on how the gunman could get so close to the Republican presidential nominee when he was supposed to be carefully guarded, and why Trump was allowed to take the stage after local law enforcement had identified Crooks as suspicious."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 Rep. Alexandria Ocasio-Cortez, D-New York.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." Local law enforcement took a photograph of Crooks and shared it after seeing him acting suspiciously outside the security perimeter, but he wasn't deemed to be a "threat" until seconds before he opened fire, she said."An individual with a backpack is not a threat," Cheatle said. "An individual with a rangefinder is not a threat."Cheatle said local enforcement officers were inside the building from which Crooks fired. But when asked why there were no agents on the roof 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, one of the Democrats who joined the calls for Cheatle to resign, noted that the Secret Service director who presided over the agency when there was an attempted assassination of former Republican 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 rally.Cheatle said the agency hopes to have its internal investigation completed in 60 days. Homeland Security Secretary Alejandro Mayorkas has separately appointed a bipartisan, independent panel to review the assassination attempt, while the department's inspector general has opened three investigations.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 Monday there were "no assets denied" for the Pennsylvania rally.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.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.Cheatle took over two years ago as head of the Secret Service's 7,800 special agents, uniformed officers and other staffers whose main purpose is protecting presidents, vice presidents, their families, former presidents and others. In announcing her appointment, Biden said Cheatle had served on his vice presidential detail and called her a "distinguished law enforcement professional with exceptional leadership skills" who had his "complete trust."Cheatle took the reins from James M. Murray as multiple congressional committees and an internal watchdog investigated missing text messages from when Trump supporters stormed the U.S. Capitol on Jan. 6, 2021. The Secret Service says they were purged during a technology transition.