The Secret Service director was before a congressional committee on Monday morning as calls mount for her to resign over security failures at a rally where a 20-year-old gunman attempted to assassinate former president Donald Trump.

The House's oversight committee hearing, led by Republicans, will be Kimberly Cheatle's first appearance before lawmakers since the July 13 Pennsylvania rally shooting that left one spectator dead. Trump was wounded in the ear and two other attendees were injured after Thomas Matthew Crooks climbed atop the roof of a nearby building and opened fire.

Lawmakers have been expressing anger over how the gunman could get so close to the Republican presidential nominee when he was supposed to be carefully guarded. The Secret Service has acknowledged it denied some requests by Trump's campaign for increased security at his events in the years before the assassination attempt.

Homeland Security Secretary Alejandro Mayorkas has called what happened a "failure," while several lawmakers including Republican House Speaker Mike Johnson said they believe Cheatle should resign. James Comer of Kentucky, leading the Republicans at the oversight committee, did so as well to begin Monday's hearing.

The Secret Service has said Cheatle does not intend to step down. So far, she retains the support of U.S. President Joe Biden, a Democrat, and Mayorkas.

Before the shooting, local law enforcement had noticed Crooks pacing around the edges of the rally, peering into the lens of a rangefinder toward the rooftops behind the stage where the president later stood, officials have told The Associated Press. An image of Crooks was circulated by officers stationed outside the security perimeter.

Witnesses later saw him climbing up the side of a squat manufacturing building that was within 135 metres from the stage. He then set up his AR-style rifle and lay on the rooftop, a detonator in his pocket to set off crude explosive devices that were stashed in his car parked nearby.

Trump injuries still unclear

The attack on Trump was the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981. It was the latest in a series of security lapses by the agency that has drawn investigations and public scrutiny over the years.

Authorities have been hunting for clues into what motivated Crooks, but so far have not found any ideological bent that could help explain his actions. Investigators who searched his phone found photos of Trump, Biden and other senior government officials, and also found that he had looked up the dates for the Democratic National Conventional as well as Trump's appearances. He also searched for information about major depressive order.

Cheatle was appointed by Biden in late 2022, though the agency also faced criticism more than once during the Trump term. The Secret Service came under scrutiny after a Chinese woman was able to get through security checks at Trump's Mar-a-Lago resort in Florida.

Democrats and others have raised concerns about the lack of information to be released publicly about shooting. The Secret Service did not take part in media briefings in the hours after the shooting, as the FBI and local law enforcement officials did.

WATCH | Former Secret Service agent Briant Gant speaks to CBC about July 13 failings: 

'The burden of a secret service agent ... is to be perfect,' says former agent

Briant Gant, a former member of the U.S. Secret Service, says an event like the assassination attempt on former U.S. president Donald Trump is the agency's 'worst nightmare.' He says the independent review of the agency's actions ordered by President Joe Biden should reveal the Secret Service's original security plan for the building the gunman shot from and how he gained access to it.

Dr. Sanjay Gupta, a neurosurgeon who appears on air for CNN, wrote on Friday that "a full public assessment of Trump's injuries is necessary, for both the former president's own health and the clarity it can provide for voters about the recovery of the man who could become president of the United States once again."

"The concern is that gunshot blasts near the head can cause injuries that aren't immediately noticeable, such as bleeding in or on the brain, damage to the inner ear or even psychological trauma," Gupta wrote.