The gunman who attempted to kill former President Donald Trump was Thomas Matthew Crooks, a 20-year-old who lived south of Pittsburgh, who had been bullied at school and had political leanings that are difficult to pin down.

Crooks opened fire at Mr. Trump at a campaign rally on Saturday night from the rooftop of a building less than 150 yards from the event stage in Butler County, Pennsylvania, where the presumptive Republican nominee was speaking. Crooks was quickly shot and killed by Secret Service agents.

Law enforcement recovered an AR-style rifle at the scene of the shooting and bomb-making materials were found in the gunman’s car and home, a source told The Associated Press, which also reported Crooks had no record of military service.

Crooks’ hometown of Bethel Park is about 10 miles south of Pittsburgh and 50 miles from the shooting site in Butler. Law enforcement swarmed his home and closed off his street, according to KDKA-TV in Pittsburgh.

Neighbors told reporters they had been evacuated from their homes, in part because there was a bomb investigation.

Authorities said Crooks was not carrying an ID at the time of the shooting and he was identified through DNA. He was a 2022 graduate of Bethel Park High School, the school confirmed in a statement.

“The school district wishes to express its sincere wishes for a speedy and full recovery for Mr. Trump and for those in attendance at the Saturday event who may have been physically harmed or emotionally impacted by these tragic events. We offer special condolences to the family of at least one attendee who was killed,” the statement said. “Our school district will cooperate fully with the active law enforcement investigation surrounding this case, and as such, we are limited in what we can publicly disclose.”

Jason Kohler, who said he attended the same high school but did not share any classes with Crooks, said Crooks was bullied at school and sat alone at lunchtime. Other students mocked him for the way he dressed, such as hunting outfits, Mr. Kohler said.

“He was bullied almost every day,” Mr. Kohler told reporters. “He was just an outcast, and you know how kids are nowadays.”

Images of Crooks’ body reviewed by AP show that at the time of the shooting, he appears to have been wearing a T-shirt from Demolition Ranch, a popular YouTube channel with more than 11.6 million subscribers that regularly posts videos that show creator Matt Carriker firing off handguns and assault rifles at targets that include human mannequins and vehicles.

Mr. Carriker, who lives in Texas, did not respond to a phone message or email on Sunday, but posted a photo of Crooks’ bloody corpse wearing his brand’s T-shirts on social media with the comment “What the hell.”

Authorities are trying to establish Crooks’ motive.

His political leanings were a bit unclear.  

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

Asked if law enforcement did not know the shooter was on the roof until he began firing, Kevin Rojek, special agent in charge of the FBI Pittsburgh field office, responded, “That is our assessment at this time.”

“It is surprising” that the shooter was able to get off as many rounds as were fired, Mr. Rojek said at a press conference. He added that “all the details of that will come out later [in the] investigation.”

Mr. Trump was struck in the right ear and is reported by aides to be “fine.” He was walking unaided when he arrived in New Jersey early Sunday.

One rally attendee was killed and two others were critically injured, the Secret Service said.

Family members identified the person killed as Corey Comperatore, a former fire chief of Buffalo Township in Pennsylvania.

Dawn Comperatore Schafer, posting on Facebook, said the rally “claimed the life of my brother, Corey Comperatore.”

“The hatred for one man took the life of the one man we loved the most,” she wrote, adding her brother had just turned 50.

Pennsylvania Gov. Josh Shapiro, a Democrat, said he spoke to Mr. Comperatore’s family. They said before he died, Mr. Comperatore dove onto family members to protect them.

“Corey was the very best of us. May his memory be a blessing,” the governor said. “Last night was shocking for his community and for this commonwealth and, I know, for this country. Political disagreements can never, ever, be addressed through violence.”

Pennsylvania State Police said the Secret Service was responsible for surveying the venue where the rally was held. Asked whether there was anything about the venue that made it particularly difficult to secure, Pennsylvania State Police Lt. Col. George Bivens deferred to the Secret Service, which was not present at the news conference.

A Trump supporter said he saw the man with a rifle crawling on top of the roof of a building behind the campaign rally stage. According to the eyewitness, as reported to the BBC, rally-goers outside the event spotted the man “bear crawling” across the roof, carrying a rifle.

The witnesses frantically alerted law enforcement and were surprised when Mr. Trump was not immediately ushered from the stage.

“We’re pointing at the guy, crawling up the roof,” the witness said. “He had a rifle, you could clearly see he had a rifle. I’m thinking to myself, why is Trump still speaking? Why have they not pulled him off the stage?”

The witness said he alerted the Secret Service on the roof to the crawling man with the rifle, but Mr. Trump was not removed from the stage.

“The next thing you know, five shots rang out,” the witness said, adding he is “100%” certain the shots came from the rifle of the man crawling across the roof.

• This article is based in part on wire service reports.

• Dave Boyer can be reached at dboyer@washingtontimes.com.

• Tom Howell Jr. can be reached at thowell@washingtontimes.com.