Whistleblowers inside the Department of Homeland Security have alleged that the majority of the security detail for former President Donald Trump were "not even Secret Service," according to a Republican lawmaker.

Sen. Josh Hawley of Missouri relayed these claims in a public letter to Secretary of Homeland Security Alejandro Mayorkas on Friday outlining a series of security failures at Trump's fateful Butler, Pennsylvania, rally.

"Whistleblowers who have direct knowledge of the event have approached my office. According to the allegations, the July 13 rally was considered to be a 'loose' security event," Hawley wrote in the letter. "For example, detection canines were not used to monitor entry and detect threats in the usual manner. Individuals without proper designations were able to gain access to backstage areas."

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Other lapses in security protocol allegedly included a lack of personnel stationed around the security perimeter and an inadequetly enforced buffer zone around the podium.

Among the most troubling is the claim that the majority of personnel protecting the former president were not U.S. Secret Service (USSS) agents.

"Whistleblower allegations suggest the majority of DHS officials were not in fact USSS agents but instead drawn from the department's Homeland Security Investigations (HSI)," Hawley wrote. "This is especially concerning given that HSI agents were unfamiliar with standard protocols typically used at these types of events, according to the allegations."

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The Missouri senator criticized the DHS for failing to provide information about the incident to Congress and "abruptly ending the only call with USSSS before most senators could even ask a question."

Sen. Ron Johnson, R-Wis., previously detailed the Secret Service briefing given to senators on Wednesday about the recent assassination attempt against former President Trump, saying there had been "virtually no information" provided. 

"The director of the Secret Service did admit there were mistakes and gaffes," Johnson said, referring to Kimberly Cheatle. But the briefing, which was given by a separate official, "was largely irrelevant," according to Johnson. Only four senators were allowed to ask questions and there were no follow-ups, he said. 

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Hawley's letter demands answers to a series of questions relevant to the claims made by the whistleblowers, including the ratio of USSS to HSI agents and pre-rally security investigations.

Fox News Digital's Julia Johnson contributed to this report.