BETHEL PARK, Pa. — Federal investigators have determined that the rifle used in an assassination attempt on former president Donald Trump on Saturday was legally purchased by the gunman’s father in 2013, according to a person familiar with the investigation.

The 20-year-old gunman, Thomas Matthew Crooks, bought 50 rounds of ammunition at a gun store the morning of the shooting, according to this person, who spoke on the condition of anonymity to share information that has not been publicly released.

The FBI said Monday that agents have accessed the data on the cellphone belonging to Crooks, and technical experts “continue to analyze his electronic devices,” the agency said in a statement.

Officials are trying to determine why Crooks opened fire at the rally for Trump in Butler, Pa., wounding the former president and leaving one rallygoer dead and two others critically injured. Even with access to the gunman’s family and friends, his phone and the phones of some of the people he communicated with, investigators have found little information pointing to a motive, according to people familiar with the investigation who spoke on the condition of anonymity to discuss an ongoing investigation.

FBI officials have said they do not have meaningful evidence of an ideology driving Crooks to commit the act. He appeared to act alone, they said. Officials cautioned that the investigation was in an early stage and that their understanding could change with more evidence.

President Biden and Vice President Harris were briefed Monday in the Situation Room by senior law enforcement and security officials, including FBI Director Christopher A. Wray, Attorney General Merrick Garland and Homeland Security Secretary Alejandro Mayorkas.

Secret Service Director Kimberly Cheatle also participated. The agency faces questions over how Crooks managed to get on the roof of a building outside the rally’s security perimeter and open fire. Video taken just before the attack appears to show some spectators trying to alert police to the shooter’s presence.

FBI special agent in charge Kevin Rojek said Crooks used an AR-style rifle chambered in 5.56mm, a common caliber for such weapons. Authorities said the weapon was identified and traced using records from a gun dealership that is no longer operating. On the morning of the shooting, Crooks bought 50 rounds of ammunition at a local gun store called Allegheny Arms, according to the person familiar with the investigation. The store owner and employees declined to comment when visited by a reporter from The Washington Post on Monday morning. They offered a statement that said they have a “prerogative to cooperate with law enforcement in every way.”

“We are thankful that President Trump was not assassinated and our hearts and prayers go out to all victims of this horrible incident,” the statement said.

Crooks lived with his parents in Bethel Park, a quiet suburb of Pittsburgh that is about 50 miles south of where Saturday’s rally took place. He worked at a nursing home and graduated in May with an associate’s degree in engineering science from the Community College of Allegheny County in western Pennsylvania, according to a college spokesperson.

“Like all Americans, we are shocked and saddened by the horrific turn of events that took place,” the community college said in a statement. It expressed relief that Trump is safe and offered condolences to the family of Corey Comperatore, the 50-year-old engineer, firefighter and father of two who was killed.

Crooks planned to enroll at Robert Morris University in Pittsburgh this fall, according to a university spokesperson. At the community college, Crooks was a part of a proof-based mathematics book club that met weekly, according to a former classmate who spoke on the condition of anonymity to protect his privacy. While Crooks was interested in math, the classmate said, he took more of a liking to physics.

The classmate said Crooks was a reserved person, rarely talking about politics at school and politically leaning more moderate to central right. When the classmate heard Crooks was identified as the gunman, he texted Crooks saying he was sorry someone was trying to impersonate him. He didn’t receive a text back. Then, his classmate saw the photo of Crooks fatally shot on the roof of a building.

“If I had to speculate, I would assume this was a suicide-by-cop,” said the classmate, who considered Crooks a friend. “Tom was a good guy, I assumed he was going to live a good life. I don’t know what drove him to do this. I worry about our democracy, his family, the Crooks. I feel awful for the firefighter’s family.”

In Crooks’s hometown of Bethel Park, residents on Monday had grown weary of the journalists who had descended on their community in search of answers about the would-be assassin.

The American flag outside of the Sheetz convenience store billowed at half-staff. And next to a shuttered heating and cooling company, someone planted a sign that said, “TRUMP LIVES!” Beside it was a picture of the former president, ear bloody and fist raised.

Some store owners preprinted statements to hand to visiting reporters. At the library, someone had cut out Crooks’ picture from a high school yearbook. Residents largely declined to talk about Crooks but defended Bethel Park as home to kindhearted and community-oriented people, despite the gunshots that had landed their hometown at the center of American history.

Police and volunteer firefighters blocked off access to the gunman’s neighborhood on Sunday, but the area was reopened to the public Monday. Crooks lived in a small, one-story brick home with his parents, Matthew and Mary, who have so far declined to publicly comment on the attack.

The Crooks have owned their 1,000-square-foot ranch-style home since 1998, the same year they married and five years before their son Thomas was born.

Matthew and Mary Crooks have both been licensed counselors for over two decades, Pennsylvania state records show, a certification that requires a master’s degree or higher. A LinkedIn profile lists Matthew Crooks’ employer as a company that provides mental health care, but the company did not respond to requests for comment. Matthew Crooks also owns a business called C&F Professional Disability Services based out of his mother-in-law’s Pittsburgh home, records show. The company has a minimal online footprint.

Up and down the block where the Crooks lived, many neighbors did not open their doors, or they declined to comment. Liam Campbell, who lives across street, said he and his family were rustled out of bed around midnight Saturday by police who were evacuating the neighborhood for fear that there might be an explosive device inside Crooks’s house.

“They said there was a device in a neighboring household that needed to be removed and that just we need to get everyone out of the house out and stay somewhere,” said Campbell, 17, adding that he was not allowed to return home until 10 p.m. Sunday. During Campbell’s freshman year of high school, he and Crooks rode the school bus together. But, Campbell said, Crooks “kept to himself” and didn’t seem to interact with many people at school or in the neighborhood.

“He would just walk around the neighborhood by himself,” Campbell said. “He was a quiet kid, and he was kind of strange. He sat by himself. Didn’t talk to anyone. … But he just seemed like a normal person who just didn’t like talking to people.”

The gunman was a member of the Clairton Sportsmen’s Club, a shooting club in Clairton, Pa., a club lawyer confirmed Monday. The club “fully admonishes the senseless act of violence that occurred on Saturday,” Robert S. Bootay III said. He added that the club “offers its sincerest condolences to the Comperatore family and extends prayers to all of those injured, including the former president.”

Barrett and Hilton reported from Washington. Bailey reported from Atlanta. Shawn Boburg, Alice Crites, Monika Mathur, Razzan Nakhlawi, María Luisa Paúl, Aaron Schaffer, Perry Stein and Matt Viser in Washington contributed to this report.