WASHINGTON (AP) — The 20-year-old nursing-home employee from suburban Pittsburgh who tried to assassinate former President Donald Trump was a registered Republican who packed explosives in the vehicle he drove to the campaign rally an hour from his home.

Law enforcement officials were working Sunday to learn more about Thomas Matthew Crooks of Bethel Park, Pennsylvania, to determine what motivated him to open fire on the rally from a nearby rooftop, killing one spectator, before he was shot dead by the Secret Service.

The FBI said Sunday it has not yet identified any underlying ideology or threatening writing or social media posts from Crooks, who graduated from high school two years ago and had no past criminal cases against him, according to public court records. The FBI believes he acted alone.

Trump, the presumptive Republican presidential nominee, said on social media the upper part of his right ear was pierced in the shooting. Two spectators were critically injured, authorities said. The man killed was Corey Comperatore, a former fire chief from the area who Pennsylvania's governor says died a "hero" by diving onto his family to protect them.

Relatives of Crooks didn't immediately respond to messages from The Associated Press. His father, Matthew Crooks, told CNN late Saturday that he was trying to figure out "what the hell is going on" but wouldn't speak about his son until after he talked to law enforcement. An FBI official told reporters that Crooks' family is cooperating with investigators.

Crooks graduated from Bethel Park High School in 2022. In a video of the school's graduation ceremony posted online, Crooks can be seen crossing the stage to receive his diploma, appearing slight of build and wearing glasses. The school district said it will cooperate fully with investigators. His senior year, Crooks was among several students given an award for math and science, according to a Tribune-Review story at the time.

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," Kohler told reporters. "He was just a outcast, and you know how kids are nowadays."

Crooks worked at a nursing home as a dietary aide, a job that generally involves food preparation. Marcie Grimm, the administrator of Bethel Park Skilled Nursing and Rehabilitation, said in a statement she was "shocked and saddened to learn of his involvement." Grimm added that Crooks had a clean background check when he was hired.

Crooks' political leanings were not immediately clear. Records show Crooks was registered as a Republican voter in Pennsylvania, but federal campaign finance reports also show he gave $15 to a progressive political action committee on Jan. 20, 2021, the day President Joe Biden was sworn into office.

Butler County district attorney Richard Goldinger told AP on Sunday 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 Sunday 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 neighborhood.

Bomb-making materials were found inside Crooks' vehicle near the Trump rally and at his home, according to two officials who were not authorized to speak publicly and spoke on condition of anonymity. A white Allegheny County Police truck identified as bomb squad pulled up to the home late Sunday morning.

Crooks used an AR-style rifle, which authorities said they believe was purchased by his father. Kevin Rojek, FBI special agent in charge in Pittsburgh, said that investigators do not yet know if he took the gun without his father's permission.

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 the officials, who spoke to AP on condition of anonymity to discuss an ongoing investigation.

A video posted to social media and geolocated by AP 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.

The roof where Crooks lay was less than 150 meters (164 yards) from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. That is a distance at which U.S. Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle.

Images of Crooks' body reviewed by AP show he appears to have been wearing a T-shirt from Demolition Ranch, a popular YouTube channel that regularly posts videos of its creator firing off handguns and assault rifles at targets that include human mannequins.

Matt Carriker, the Texas-based creator of Demolition Ranch, did not respond to a phone message or email on Sunday, but posted a photo of Crooks' bloody corpse wearing his brand's T-shirt on social media with the comment "What the hell."

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Associated Press journalists Marc Levy in Harrisburg, Pennsylvania, Julie Smyth and Joshua Bickel in Bethel Park, Michael R. Sisak in New York, Michael Kunzelman in Silver Spring Maryland, Mike Balsamo in Chicago and Colleen Long in Washington contributed.