The FBI identified Thomas Matthew Crooks as the suspected shooter in the early hours of Sunday morning, and had determined by Monday afternoon that he is believed to have acted alone in what is being investigated as a case of possible domestic terrorism.

Law enforcement officials found explosive material allegedly belonging to Crooks, which were found after multiple reports of suspicious packages around the shooter's location, The Wall Street Journal and CNN reported.

CNN also reported explosive material was found in Crooks’ residence.

Crooks was shot and killed by Secret Service after firing shots at the Trump campaign event from a nearby building that was outside the rally and an AR-style rifle was recovered at the scene, authorities said.

Crooks was found by a law enforcement officer on the roof of a nearby building moments before shooting at Trump, and pointed his rifle at the officer, the Associated Press reported citing an anonymous source, who retreated off the roof moments before the Secret Service fired on Crooks.

Crooks was a 2022 graduate of Bethel Park High School and registered Republican with no criminal history, The New York Times reported.

He donated $15 to the Progressive Turnout Project, a Democratic voter contact campaign in 2021, but did not seem to be otherwise politically involved.

He was employed by the Bethel Park Skilled Nursing and Rehabilitation Center as a dietary aid, according to administrator Marcie Grimm, who said Crooks "performed his job without concern and his background check was clean."

At age 20, Crooks was not old enough to have voted in a presidential election before.

Crooks was identified as the shooter using his DNA and biometric information, law enforcement officials said.

CNN reached Crooks’ father, Matthew Crooks, on Saturday, but the man said he would not speak to the public about his son until he figured out “what the hell is going on” and spoke to law enforcement.