FIRST ON FOX: Rep. Andy Biggs, R-Ariz., is calling on U.S. Secret Service Director Kimberly Cheatle to back up her testimony in Monday’s explosive hearing with hard proof.

Biggs sent a letter to Cheatle shortly after her House Oversight Committee hearing concluded, asking her to turn over documentation related to safety measures, security perimeters, personnel and other notes related to law enforcement’s preparation for former President Trump’s July 13 rally in Butler, Pennsylvania.

A 20-year-old gunman opened fire at the event, killing one attendee and wounding two others. The shooter was killed by Secret Service snipers.

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Andy Biggs, Kimberly Cheatle

Rep. Andy Biggs is pressuring Secret Service Director Kimberly Cheatle for more information on what could have led to security lapses at Donald Trump's Pennsylvania rally. (Getty Images)

Trump himself was shot in the upper part of his right ear and was rushed offstage by Secret Service agents.

"As you are aware, The United States House of Representatives is investigating the United States Secret Service’s…preparation for, and response to the attempted assassination of President Donald J. Trump on July 13, 2024, which resulted in the death of Corey Comperatore, serious injury to two other attendees, and nearly resulted in the first assassination of a candidate for President in nearly 60 years," Biggs wrote to Cheatle.

"It is unfathomable that an assassin was able to gain access to and fire shots at President Trump from an unsecure building with a direct line of sight to the rally stage. It is equally unfathomable that public reporting suggests that U.S. Secret Service had identified the building as a potential vulnerability and failed to include that vulnerability within the security perimeter or otherwise ensure its security."

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Trump holds fist

Republican presidential candidate Donald Trump is seen with blood on his face and surrounded by Secret Service agents after a failed assassination attempt during his campaign rally in Butler, Pennsylvania, on July 13, 2024. (Rebecca Droke/AFP via Getty Images)

Biggs accused the Secret Service of having "failed its no-fail mission" that day.

It comes after a marathon hearing on Monday in which lawmakers on both sides of the aisle expressed frustration with Cheatle’s apparent lack of candor in her answers. At least two House Democrats are now calling on her to resign.

Biggs was among those who told Cheatle point-blank to step down from her role: "You should have come today ready to give us answers. I call upon you to resign today."

Fox News Digital reached out to the Secret Service for comment.

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Kimberly Cheatle attends a House of Representatives Oversight Committee hearing

Both Republicans and Democrats were unsatisfied with Secret Service Director Kimberly Cheatle's answers during Monday's hearing. (Reuters/Kevin Mohatt)

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The letter comes as the House gears up for an expected vote on establishing a bipartisan commission to investigate the shooting sometime this week.

It's also being sent a day after Homeland Security Secretary Alejandro Mayorkas announced a 45-day independent review of the assassination attempt against Trump.