Editor's note: This is a developing story and will be updated throughout the day.

The Secret Service is under growing pressure to explain what went wrong in the hours and minutes before a gunman opened fire in an attempted assassination on former President Donald Trump at his Pennsylvania rally on Saturday.

One attendee was killed and two were critically injured before a sniper fatally shot 20-year-old Thomas Matthew Crooks on the roof of a building about 400 feet from the rally stage.

"The Secret Service is working with all involved federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again," Secret Service Director Kimberly Cheatle said in a statement Monday.

The Secret Service is not substantially changing its security plan for the RNC, which takes place from Monday to Thursday, Cheatle said, despite the Saturday shooting.

"I am confident in the security plan our Secret Service RNC coordinator and our partners have put in place, which we have reviewed and strengthened in the wake of Saturday's shooting," Cheatle said.

On Sunday, Secret Service RNC coordinator Audrey Gibson-Cicchino also stood by the convention's security plan, which she said had been developed over 18 months and given the highest level of security designation.

She added firearms are not allowed within the convention venue or the Secret Service's wider security perimeter.

Wisconsin is an open-carry state, however, which means firearms are allowed just blocks away from the RNC, outside of the Secret Service's cordoned area.

The Federal Bureau of Investigation is still in the middle of its post-mortem on the Trump rally.

According to a Monday update, the FBI finished the search of Crooks' home and vehicle and have so far conducted nearly 100 interviews with law enforcement personnel, rally attendees and other witnesses.

On Sunday, FBI officials said they so far believe the shooter acted alone.

FBI specialists gained access to Crooks' phone and are analyzing his electronic devices. Based on a preliminary analysis of Crooks' phone, the FBI still did not have enough information to identify his motive, a senior U.S. law enforcement official told NBC News on Monday.

President Joe Biden has also directed an independent review in coordination with the FBI's criminal probe.

As the investigation continues and questions linger, donations have poured in to support the shooting victims, including from high-dollar donors.

An online fundraiser for the victims set up by the Trump campaign had raised upward of $4 million by Monday from more than 50,000 donors. The top donor list includes Citadel CEO Ken Griffin who made two donations totaling $100,000, a Citadel spokesperson confirmed to CNBC. Pershing Square CEO Bill Ackman also confirmed his $10,000 donation to CNBC.

"We have committed the full force of the FBI to this investigation," FBI Director Christopher Wray said on a call with reporters Sunday. "The American people can rest assured that we will leave no stone unturned as we work to get to the bottom of what happened."

The Secret Service and other law enforcement agencies have been bombarded with questions about the potential security lapses that could have contributed to the circumstances around Saturday's shooting.

Later Monday, all 10 Republican senators on the Senate Judiciary Committee requested a public congressional hearing with Cheatle, Homeland Security Secretary Alejandro Mayorkas and FBI Director Christopher Wray in a letter to their committee chair, Democratic Sen. Dick Durbin of Illinois.

They were the latest in a growing tally of lawmakers calling for transparency on what law enforcement officials have uncovered about the events of Saturday's rally.

House Oversight Committee Chair James Comer, R-Ky., on Saturday requested that Cheatle testify at a congressional hearing on Monday, July 22.

"There are many questions and Americans demand answers," Comer said in a statement Saturday night, just hours after the shooting took place.

Comer was followed by House Homeland Security Committee Chair Mark Green, R-Tenn., who said he would launch his own investigation into how the Secret Service and the Department of Homeland Security coordinated the security procedures.

"The seriousness of this security failure and chilling moment in our nation's history cannot be understated," Green wrote in a letter to Secretary Mayorkas on Sunday.

Green's letter also repeated a rumor that the Department of Homeland Security rejected Trump's requests for additional security resources in the weeks ahead of Saturday's rally. Both the Secret Service and Mayorkas have staunchly rejected that claim.

"That is an unequivocally false assertion," Mayorkas said Monday on ABC's "Good Morning America." "We had enhanced security for the former president beginning at least in June. We have not received any requests for additional security measures that were rebuffed."