The director of the Secret Service says the assassination attempt on Donald Trump was the “most significant operational failure” in decades.
Ms Cheatle told the House of Representatives Oversight and Accountability Committee she took “full responsibility for any security lapse”.
The shooting at an outdoor campaign rally in Butler, Pennsylvania, wounded Trump’s right ear, killed one audience member and injured another on 13 July.
Latest as Secret Service chief grilled over Trump shooting
Thomas Crooks, the 20-year-old nursing home aide suspected of the shooting, was killed by law enforcement.
The fact the suspect was able to get within range of Trump on the rooftop of a nearby building has angered lawmakers who blame security lapses in the Secret Service.
Members of the public had pointed out a suspect to law enforcement before the shooting.
“We failed. As the director of the United States Secret Service, I take full responsibility for any security lapse,” Ms Cheatle said in her testimony before the committee.
“The assassination attempt on former President Donald Trump on 13 July is the most significant operational failure at the Secret Service in decades.”
In the face of Republican claims the agency denied resources to protect Trump, she said security for the former president had grown ahead of the shooting.
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“The level of security provided for the former president increased well before the campaign and has been steadily increasing as threats evolve,” she said.
“Our mission is not political. It is literally a matter of life and death.”
She wouldn’t answer specific questions about the day’s security plan from Republicans and Democrats, adding the matter is being investigated internally.
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Trump shooting ‘most significant failure’
Before the shooting, local officers had noticed Crooks pacing around the edges of the rally, officials have told The Associated Press.
An image of Crooks was even circulated by officers stationed outside the security perimeter.
Asked if Crooks was identified as suspicious, Ms Cheatle said he was – but a threat was not confirmed.
“If the detail had been passed information that there was a threat, the detail would never have brought the former president onto the stage,” she added.
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Police snipers return fire after shots were fired at Trump’s rally. Pic: AP
The House Judiciary Committee claims it has evidence the agency was not properly resourced for Trump’s rally because of staffing shortages created by a rival event in the same state and a NATO summit days before in Washington.
But so far, Ms Cheatle has defied calls for her departure.
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“Continuity of operations is paramount during a critical incident and US Secret Service Director Kimberly Cheatle has no intentions to step down,” said spokesperson Anthony Guglielmi.
“She deeply respects members of Congress and is fiercely committed to transparency in leading the Secret Service through the internal investigation and strengthening the agency through lessons learned in these important internal and external reviews.”