FBI agents are going door to door in a Bethel Park, Pennsylvania, neighborhood as they search for information about the gunman who shot at former President Donald Trump at a rally over the weekend.

The bureau announced in a statement Sunday night that its investigation of the assassination attempt suggests Thomas Matthew Crooks, 20, acted alone when he fired into a crowd at the Butler Farm Show Grounds, killing one, critically injuring two, and causing a minor injury to Trump.

The FBI said it has not identified Crooks’s motive. Agents have approached people in Crooks’s neighborhood, situated in a suburb about an hour from the rally location, on Monday morning as they seek information about him, CBS News reported.

The FBI’s investigation is being conducted out of its Pittsburgh Field Office. The bureau said it is “working to determine the sequence of events and the shooter’s movements prior to the shooting, collecting and reviewing evidence, conducting interviews, and following up on all leads.”

Crooks was killed by a Secret Service counter-sniper as soon as he opened fire on the rally crowd. Little is known about him. He had a limited presence on social media, and reports indicate he was a reserved student in high school. The bureau said it searched Crooks’s home and was working to examine his phone, which is in the FBI’s possession.

Questions swirl about what security failures led to Crooks having a clear shot at Trump from the rooftop of a nearby building. The Secret Service identified the roof the gunman shot from as a “potential vulnerability” in the days before the rally, according to NBC News.

Crooks purchased roughly 50 rounds of ammunition at a local gun store hours before the attack, a law enforcement official told CNN.

Bomb technicians identified “suspicious devices” in Crooks’s home and vehicle that were sent to an FBI laboratory for testing, the FBI said.

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While the FBI continues to lead the investigation of the shooting, congressional lawmakers have also opened inquiries into the matter.

The Secret Service is set to brief the House Oversight Committee about the incident on Tuesday, and Director Kimberly Cheatle is set to appear for a hearing on July 22, according to a committee aide. FBI Deputy Director Paul Abbate spoke with leaders of the House Judiciary Committee on Monday and also provided a briefing on the incident, according to a source familiar with the call.