BETHEL PARK, Pa. — The FBI said Sunday that investigators have not yet identified any ideology fueling the gunman who fired at former president Donald Trump at a packed campaign rally, and they believe he carried out the horrifying assassination attempt on his own.

The gunman was identified as 20-year-old Thomas Matthew Crooks, of Bethel Park, a strong math student in high school who was working as a nursing home employee.

Seconds after he opened fire from a rooftop outside the rally’s security perimeter Saturday evening, using an AR-style rifle that was legally purchased by his father, he was fatally shot by the Secret Service, authorities said.

“At this time, the information that we have indicates that the shooter acted alone and that there are currently no public safety concerns,” FBI special agent in charge Kevin Rojek said in a telephone briefing. “At present we have not identified an ideology associated with the subject, but I want to remind everyone that we’re still very early in this investigation.”

Officials said they had not reached any final conclusions and are still scrutinizing Crooks’ associates to see if anyone aided his violent plan.

The wounding of a former president and presumptive nominee at the outdoor event in Butler, Pa., shocked the nation and brought calls for calmer political rhetoric on the eve of the Republican National Convention in Milwaukee. It also sparked significant questions about what security lapses allowed the gunman to get onto the roof undetected.

At the White House, President Biden pledged that every measure would be taken to get to the bottom of the attack, and he urged the public to give the FBI time to properly investigate before reaching conclusions.

Speaking from the Oval Office on Sunday night, Biden said the country needs “to lower the temperature … Politics must never be a literal battlefield or, god forbid, a literal killing field.”

While authorities have not given details of Trump’s injuries, the former president said in a post on Truth Social that he was “shot with a bullet that pierced the upper part of my right ear.”

The bullets that missed Trump still proved deadly. One spectator was killed, and two others were critically injured in the attack.

Rojek identified the firearm used in the attack as an AR-style rifle chambered in 5.56 mm, a common caliber for such weapons. The gunman’s family, the FBI said, is cooperating with investigators. Other law enforcement officials said the weapon helped investigators identify the dead gunman after ATF agents quickly tracked down the gun purchase paperwork.

During a search of a vehicle used by the gunman, agents found a “rudimentary” suspected explosive device, officials said. “We have seized the device, rendered it safe and we are also in the process of analyzing that further,” Rojek said. In a later update, the FBI said suspicious devices had been found in his home and vehicle, and both had been rendered safe by bomb technicians.

FBI officials said they have reviewed some of the gunman’s recent electronic communications and hope his cellphone will provide more such evidence.

Officials said he did not have a history of interactions with law enforcement or of mental illness, and they had yet to find any accounts of him making threats. Former classmates described him as kind, polite and smart.

Lacking a clear sign of his motive, investigators searched for every scrap of potential evidence. The shooter’s phone was sent to technical experts at FBI labs in Quantico, Va. His vehicle was towed to undergo further examination. And his online activity, which so far appears sparse and inconclusive, will be carefully examined.

“We are investigating this as an assassination attempt but also looking at it as a potential domestic terrorism attack,” senior FBI official Robert Wells said. The FBI urged anyone with relevant information to submit it to the agency.

The gunman worked as a dietary aide at Bethel Park Skilled Nursing and Rehabilitation Center in the Pittsburgh suburb of about 34,000 people located about 40 miles south of Butler.

Crooks “performed his job without concern, and his background check was clean,” the administrator of the facility said in a statement.

“A quiet kid” is how Sarah D’Angelo, 20, remembered the teenager she saw before 7:30 a.m. almost every school day for years at Bethel Park High School. Organized by their last names, they sat a few chairs apart.

He would arrive on time and spend most of the 20-minute period either finishing homework or playing video games on his computer, D’Angelo recalled. They hardly talked, D’Angelo said, mostly because it was early and everyone was tired.

“He was nice to anyone he talked to,” D’Angelo said.

D’Angelo said that Crooks did not appear to have many friends, but that he also did not strike her as particularly lonely. He was good at math, “a calculus-type person,” she said. A local media outlet’s list of graduates of Bethel Park High School in 2022 listed Crooks as one of 20 students to have received a $500 prize for math and science from the school that year.

“There were a few people that were more violent in school,” D’Angelo said. “He was not one of those kids.”

On Sunday, she found herself thinking about their final project in honors American history, which was about the assassination of John F. Kennedy.

Their teacher instructed them to determine what they thought happened — including how many gunmen there were, and where they hid to fire the fatal shots.

Summer Barkley, 19, who lives in Bethel Park, remembered Crooks as a quiet, polite and intelligent student in her freshman history class. “He didn’t talk a lot, but when he did, it was never anything negative,” Barkley said.

“Our history teacher could always count on him to be able to talk about different things about history, know his facts,” she said.

Barkley said she saw Crooks at high school frequently until the pandemic hit during the spring of their sophomore year. Classes went online for the rest of the semester and then to a hybrid model.

She was with a group of friends Saturday night playing video games when she heard he had been identified as the gunman.

“Our jaws were open because we just couldn’t believe it,” she said. “I’m not excusing anything he did, but it was a shock to see someone who was nice to me in high school and got good grades and things like that to have such a horrible outcome.

“It was just something that was never something we could have seen coming. It was not at all something where we were like, ‘Okay, maybe I can see his behavior here, or something like that.’”

According to state records, Crooks was registered as a Republican, though campaign finance records show that someone with his name and street address gave $15 to the Progressive Turnout Project, a Democratic voter-turnout organization, in January 2021.

An initial review of online social media platforms revealed little about the shooter. Discord, the online gaming chat platform, said Sunday that it had removed a “rarely utilized” account that was linked to him.

“We have found no evidence that it was used to plan this incident, promote violence or discuss his political views,” said Clint Smith, Discord’s chief legal officer. Discord is cooperating with law enforcement, Smith said.

Bethel Park residents described the borough as a tightknit community made up of small-business owners and self-described family men who like to talk about their children but not politics.

The fire department consists entirely of volunteers, said Stephen Diethorn, 66, who with his wife owns Ma and Pop’s diner. “People like to help each other, and they like to get along.”

By early Sunday, authorities had sealed off the area around the shooter’s home. Fire department vehicles restricted access for several blocks, allowing only residents and investigators to enter.

In the streets outside that area, the neighborhood seemed apolitical, with no yard signs for any cause or candidate.

But Diethorn, the diner owner, said he had noticed political tension intensifying somewhat in recent years.

Last year, he had to come out from the kitchen to break up an argument between a man who supported President Biden and another who liked Trump. They were yelling. He encouraged them to “be civil.”

Then a few months ago, a man who rides his bicycle around town in a colonial outfit and with a big Trump sign decided to park outside of the diner. Diethorn asked him to move across the street. He didn’t want anything controversial to discourage business.

The separation between food and politics collapsed Sunday morning, as the TV mounted above the four pots of coffee flashed pictures of the former president bleeding from the ear next to the words “Bethel Park.”

“To see a statement talking about [Bethel Park] High School on TV, it’s insane, like out of a movie or something” said customer Tony Serkis, 51. “Unfortunately, it is scarring this community right here.”

Serkis, a lifelong conservative who works in IT, credits Trump with pushing economic policies during his first term that helped Serkis and his family. They were supposed to go to the rally Saturday, but scheduling conflicts prevented them from attending.

Serkis said he grew up with close friends who were liberal and thinks it is important for people who have differences to be able to talk to each other. “We’ve lost that,” he said. “I mean, someone tried to assassinate a former president.”

Barrett, Stein and Hilton reported from Washington. Steve Hendrix in Bethel Park, Annabelle Timsit in London and Shawn Boburg, Alex Horton and Cat Zakrzewski in Washington contributed to this report.