WASHINGTON — The 20-year-old man who tried to assassinate former President Donald Trump first came to law enforcement’s attention at Saturday’s rally when spectators noticed him acting strangely outside the campaign event.

The tip sparked a frantic search but officers were unable to find him before he managed to get on a roof, where he opened fire.

In the wake of the shooting that killed one spectator, investigators were hunting for any clues about what may have drove Thomas Matthew Crooks, of Bethel Park, Pennsylvania, to carry out the shocking attack. The FBI said they were investigating it as a potential act of domestic terrorism, but the absence of a clear ideological motive by the man shot dead by Secret Service led conspiracy theories to flourish.

“I urge everyone — everyone, please, don’t make assumptions about his motives or his affiliations,” President Joe Biden said in remarks Sunday from the White House. “Let the FBI do their job, and their partner agencies do their job. I’ve instructed that this investigation be thorough and swift.”

The FBI said it believes Crooks, who had bomb-making materials in the car he drove to the rally, acted alone. Investigators have found no threatening comments on social media accounts or ideological positions that could help explain what led him to target Trump before Secret Service rushed the presumptive Republican presidential nominee off the stage, his face smeared with blood.

Trump said on social media the upper part of his right ear was pierced in the shooting, but advisers said he was in “great spirits” ahead of his arrival Sunday in Milwaukee for the Republican National Convention. Two spectators were critically injured, while a former fire chief from the area, Corey Comperatore was killed. Pennsylvania’s governor said Comperatore, 50, died a hero by diving onto his family to protect them.

Relatives of Crooks didn’t respond to numerous messages seeking comment. 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.

Several rallygoers reported to local officers that Crooks was acting suspiciously and pacing near the magnetometers, according to a law enforcement official who spoke on the condition of anonymity because they were not authorized to discuss the investigation. Officers were then told Crooks was climbing a ladder, the official said. Officers searched for him but were unable to find him before he made it to the roof, the official added.

Butler County Sheriff Michael Slupe told the AP that a local officer climbed to the roof and encountered Crooks, who saw the officer and turned toward him just before the officer dropped down to safety. Slupe said the officer couldn’t have wielded his own gun under the circumstances. The officer retreated down the ladder, and Crooks quickly took a shot toward Trump, and that’s when Secret Service snipers shot him, according to two officials who spoke to AP on condition of anonymity to discuss an ongoing investigation.

FBI officials said Sunday that they were combing Crooks’ background and social media activities while working to get access to his phone. The chatting app Discord, a social media platform popular with people playing online games, said Crooks appears to have had an account but used it rarely and not in the last several months. There’s no evidence he used his account to promote violence or discuss his political views, a Discord spokesperson said.

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 Biden was sworn into office.

Buffalo Township Fire Chief Corey Comperatore was killed during an assassination attempt on Republican presidential candidate and former President Donald Trump in Butler, Pa., on Saturday. (Buffalo Township Volunteer Fire Company via AP)