Sen. Josh Hawley, R-Mo., said Monday that a whistleblower had come forward to his office claiming there was supposed to be a security officer posted on the roof of the building where the gunman fired from, but that that person abandoned the post due to the heat.

The whistleblower, who Hawley said has direct knowledge of the Secret Service's planning for the event, alleged "that there was supposed to be a law enforcement presence on the roof that day," Hawley wrote in a letter to Department of Homeland Security Secretary Alejandro Mayorkas posted on X. "In fact, the whistleblower alleges that at least one individual was specifically assigned to the roof for the duration of the rally, but this person abandoned his or her post due to the hot weather."

Hawley said the whistleblower also alleged that "concerns over the heat prompted law enforcement to forego patrolling" the building and instead station security personnel inside it.

Hawley's letter requests that Mayorkas address the whistleblower's allegations by Friday.

ABC News has reached out to the Secret Service for comment on the letter.

During a hearing before the House Oversight Committee on Monday, Secret Service Director Kim Cheatle said that there was "overwatch" provided for the roof, but did not elaborate, when asked why no agents were placed on the building.

"What I can tell you is that there was a plan in place to provide overwatch, and we are still looking into responsibilities and who was going to provide overwatch," Cheatle said. "But the Secret Service in general, not speaking specifically to this incident, when we are providing overwatch, whether that be through counter-snipers or other technology, prefer to have sterile rooftops."

-ABC News' Allison Pecorin

House Oversight Committee Ranking Member Jamie Raskin, D-Md., called for Secret Service Director Kim Cheatle’s resignation in his closing statement.

"I think that the director has lost the confidence at a very urgent and tender moment in the history of the country, and we need to very quickly move beyond this," he told the other members of the House Oversight Committee following Cheatle’s testimony.

House Oversight Committee Chairman James Comer took Cheatle to task about her transparency.

"I don't believe the American people tonight, when they go to bed, are going to have confidence that the Secret Service is the best it could be at protecting President Biden, at protecting the first lady, at protecting President Trump, at protecting their families," he said.

Local law enforcement alerted the Secret Service of the shooter, Cheatle testified, but she did not elaborate.

Rep. Nick Langworthy, R-N.Y., said Cheatle has put "two black eyes" on her agency and should resign.

"Many people here have called for your resignation. I've introduced a resolution here into this body that hopefully will get a vote this week, calling on the president to fire you," Langworthy said.

House Speaker Mike Johnson met with Cheatle in person ahead of her testimony Monday morning and also told her to resign.

-ABC News’ Luke Barr

House Homeland Security Committee Chairman Mark Green, R-Tenn., led a bipartisan group of members of the committee on a visit to the shooting site in Butler, Pennsylvania, on Monday.

"To actually go see the site where a crime happens is so valuable," committee member Rep. Michael McCaul, R-Texas, told reporters after their tour.

McCaul said they went to the rooftop where the gunman was to see what view he had of Trump.

"It’s way too close," he said.

After Secret Service Director Kim Cheatle told ABC News that agents were not put on the roof because of its sloped nature, committee member Rep. Carlos Gimenez, R-Fla., said after his tour, "There’s nothing unsafe about that roof."

Cheatle told the House Oversight Committee on Monday, "There was a plan in place to provide overwatch, and we are still looking into responsibilities and who was going to provide overwatch, but the Secret Service in general, not speaking specifically to this incident when we are providing overwatch, whether that be through counter snipers or other technology, prefer to have sterile rooftops."

Several members of the committee called for Cheatle’s resignation.

Secret Service Director Kim Cheatle admitted she should have been clearer in her interview with ABC News Senior Justice Correspondent Pierre Thomas last week when saying agents were not put on the building's roof because of the roof's sloped nature.

"I should have been more clear in my answer when I spoke about where we placed personnel in that interview," Cheatle told the House Oversight Committee. “What I can tell you is that there was a plan in place to provide overwatch, and we are still looking into responsibilities and who was going to provide overwatch, but the Secret Service in general, not speaking specifically to this incident when we are providing overwatch, whether that be through counter snipers or other technology, prefer to have sterile rooftops."

Cheatle also said that if the Secret Service had information there was a threat to Trump, they wouldn't have brought him on stage, but the information wasn't passed along to them. She said the Secret Service did not know the gunman had a weapon before Trump took the stage.

She also admitted that the FBI has told her that the shooter flew a drone over the rally site.

-ABC News’ Luke Barr