The Department of Homeland Security's inspector general says it's investigating the U.S. Secret Service's handling of security for former president Donald Trump on the day a gunman tried to assassinate him at a Pennsylvania rally.

The agency said in a brief notice on its website Wednesday, the objective is to evaluate the Secret Service's "process for securing former president Trump's July 13, 2024 campaign event."

President Joe Biden already had directed an independent review of the security, while congressional committees have also pledged to look into the shooting. Secret Service Director Kim Cheatle has characterized the security failure as "unacceptable" and said the agency understands the importance of the various reviews and will co-operate.

"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," she said in a statement earlier this week.

Republican House Speaker Mike Johnson told Fox News on Wednesday morning that he was "going to call for her resignation" when Congress members return to Washington next week for committee hearings.

"Obviously the buck does stop at her desk on some of this," said Johnson.

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.

The Secret Service was present during Trump's first campaign as president at a Las Vegas rally, when a young British man with documented mental health issues tried to grab a policeman's gun and reportedly told police after his arrest he intended to fire shots at the Republican. Michael Sandford was deported after serving a short sentence for the June 18, 2016 incident.

Shooter didn't make high school rifle team

Meanwhile, the 20-year-old man who came close to killing the former president in Pennsylvania with a high-velocity bullet is mostly an enigma so far.

Thomas Crooks has been described by former classmates as an intelligent loner with few friends, and he had an apparently thin social media footprint and no hints of strong political beliefs publicly expressed.

The FBI has said it cracked into Crooks's cellphone, scoured his computer, home and car, and interviewed more than 100 people.

Crooks was registered as a Republican in Pennsylvania, but federal campaign finance reports also show he gave $15 to a progressive political action committee on Jan. 20, 2021, the day Biden was sworn into office.

Crooks, with a slight build, wire-rimmed glasses and thin hair parted in the middle, went by "Tom." He was described by classmates at Bethel Park High School as smart but standoffish, often seen wearing headphones and preferring to sit alone at lunch looking at his phone.

As a freshman, Crooks had tried out for his high school rifle team but was rejected for poor marksmanship, The Associated Press previously reported.

After graduating from high school in 2022, Crooks went on to the Community College of Allegheny County, earning an associate's degree with honours in engineering science in May. He also worked at a nursing home as a dietary aide.

Through his family, he was a member of the Clairton Sportsmen's Club, a local shooting range.

"We know very little about him," club president Bill Sellitto told the AP. "That was a terrible, terrible thing that happened Saturday — that's not what we're about by any means."

Gun legally obtained

Crooks opened fire on Trump Saturday from about 135 metres from where the former president was speaking, unleashing two quick volleys of rounds at Trump with an AR-15 style rifle. His father, Matthew Crooks, bought the gun in 2013 from a retail outdoors chain.

The day before the shooting, Thomas Crooks went to the sportsman's club and practiced on the rifle range, according to a federal intelligence briefing obtained by the AP. On the day of the attack, he purchased 50 rounds of 5.56-mm ammo for his rifle from a local gun shop and drove alone to Butler, the site of the Trump rally.

He parked at a gas station about a half-kilometre from the rally. Witnesses and law enforcement officials say Crooks walked around for at least a half-hour before climbing onto the roof of a building adjacent to the Butler Farm Show grounds, where Trump was speaking. As spectators screamed for police to respond, Crooks opened fire, letting loose two quick bursts.

A Secret Service counter sniper fired back within about 15 seconds, killing Crooks with a shot to the head. Trump said this week that one bullet clipped his right ear, and that only a last-second turn of his head kept him from potentially being mortally wounded.

One of the bullets aimed toward Trump killed firefighter Corey Comperatore, 50, a spectator who was in the bleachers. Two others were seriously wounded.

Without clear insight into what drove Crooks, many on both sides of the American political divide tried to fill the void with their own partisan assumptions, evidence-free speculations and conspiracy theories in the days since the shooting.

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RNC focuses on making the U.S. safer — without gun control

The second day of the Republican National Convention was focused on safety in the U.S., with border security and immigration the key topics, but not gun control — even in the wake of an assassination attempt on Donald Trump.

Some Republicans have pointed at Democrats for labelling Trump a threat to democracy. A Biden campaign document earlier this month said it was "time to put Trump in a bullseye." While it was a reference to focusing on his policies, Biden apologized for it in an NBC interview on Monday, and there's no evidence Crooks was aware of it.

Democrats, in turn, pointed to Trump's long history of provocative rhetoric. Trump's language describing migrants was echoed in manifestos written by mass shooters in Pittsburgh and Texas. Trump has also referred to arrested and convicted Jan. 6, 2021 rioters as "political prisoners."