The portrait pieced together so far of the 20-year-old nursing home aide who allegedly tried to assassinate Donald Trump at an election rally reveals frustratingly little about why he would make such an attempt – or how he managed to come so close to killing the former president.

The early details that have emerged about Thomas Matthew Crooks, who was shot dead by law enforcement, show a young man working an entry-level job near his hometown in Pennsylvania, where he graduated from high school in 2022 with a reputation as a bright but quiet classmate.

The FBI said on Sunday that his social media profile does not contain threatening language, nor have they found any history of mental health issues. They said he acted alone and have not identified a motive.

What is unique about Crooks – when compared to other recent shooters who opened fire at schools, churches, malls and parades – is that he came within inches of killing a presidential candidate.

US Secret Service agents bundle Donald Trump into an SUV as their colleagues engage his attacker. – AP

Saturday afternoon, Crooks slipped into a rooftop location 150 yards (140m) from the stage where Trump was speaking in Butler, Pennsylvania. He then began firing an AR-15-style semiautomatic rifle, purchased by his father, authorities said.

The gunfire killed a 50-year-old man, critically wounded two other spectators, and struck Trump’s ear – an assassination attempt that has further inflamed an already bitter US political divide.

The FBI said it was probing the shooting as "an assassination attempt and potential domestic terrorism".

A resident of Bethel Park, about an hour away from where the shooting occurred, Crooks was a registered Republican who would have been eligible to cast his first presidential vote in the November 5 election in which Trump is challenging President Joe Biden.

Public records show his father is a registered Republican and his mother a registered Democrat.

The suspect was a member of a local shooting club named Clairton Sportsmen's Club, the club confirmed  while condemning the shooting and calling it a "senseless act of violence".

Crooks was employed as a dietary aide at a nursing home at the time of the shooting, the home’s administrator said in a statement. Marcie Grimm, the administrator of the Bethel Park Skilled Nursing and Rehabilitation Center, said the home learned on Sunday morning that Crooks was the suspected gunman in the assassination attempt.

“We are shocked and saddened to learn of his involvement as Thomas Matthew Crooks performed his job without concern and his background check was clean,” Grimm said.

She added that the home was cooperating with law enforcement. She declined to comment further, citing the ongoing investigation.

Two years ago, Crooks graduated from the local high school, where received a $500 “star award” from the National Math and Science Initiative.

While at school, he did not show any particular interest in politics, according to a classmate, who asked not to be identified. Rather, his interests centred on building computers and playing games.

Crooks often kept to himself, the classmate said, and politics never came up. Their conversations revolved around school, the classmate said.

“He was super smart. That’s what really kind of threw me off was, this was, like, a really, really smart kid, like he excelled,” the classmate said.

“Nothing crazy ever came up in any conversation.”The classmate added that he had not seen or heard from Crooks since they graduated.

Residents near the Crooks’ home described feeling shocked and unsettled that an assassination attempt has been linked to a person from the sedate city of 33,000 people.

“Bethel Park is a pretty blue-collar type of area, and to think that somebody was that close is a little insane,” said Wes Morgan, a 42-year-old who works at an investment management company and bikes with his children on the same street as the Crooks’ residence.

The road is blocked to the Bethel Park home believed to be connected to Thomas Matthew Crooks. – AP

A couple standing on the porch of their nearby brick ranch-style home was left processing the events and spotlight on their neighbourhood.

“There’s never been a gun issue. There’s never been the police being called,” Mary Priselac, 67, said alongside her husband. “You kind of have to wonder what didn’t he get in life? What led to this extreme?”

Crooks’ gun – an AR-style 556 rifle – had been legally bought, the FBI officials said, adding that the FBI believed it had been purchased by the suspect’s father. The officials said “a suspicious device” was found in the suspect’s vehicle, which was inspected by bomb technicians and rendered safe.

His father, Matthew Crooks, told CNN that he was trying to figure out “what ... is going on” but wouldn’t speak about his son until after he talked to law enforcement.

Classmate: Suspect in Trump shooting was loner who was bullied at school. – AP

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 lunch time.

Other students mocked him for the way he dressed, such as hunting outfits, Kohler said.

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

Butler County district attorney Richard Goldinger said that Crooks had been previously unknown to investigators in his county and had not been on their radar. He said the investigation had so far not turned up any evidence that he had coordinated with anyone else in the region.

A blockade had been set up preventing traffic near Crooks’ house, which is in an enclave of modest brick houses in the hills outside blue-collar Pittsburgh and about an hour's drive from the site of the Trump rally. Police cars were stationed at an intersection near the house and officers were seen walking through the neighbourhood.

One local police officer climbed to the roof and encountered Crooks, who pointed his rifle at the officer. The officer retreated down the ladder, and Crooks quickly took a shot toward Trump, and that’s when Secret Service snipers shot him, said officials, who spoke on condition of anonymity to discuss an ongoing investigation.

A video posted to social media and geolocated shows Crooks wearing a gray t-shirt with a black American flag on the right arm lying motionless on the roof of a manufacturing plant just north of the Butler Farm Show grounds where Trump’s rally was held.

Images of Crooks’ body shows 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.