STORY: FBI agents were spotted on Monday at the home of the 20-year-old attempted assassin who opened fire on a Donald Trump rally over the weekend.

The suspect, Thomas Matthew Crooks, was killed by Secret Service sharpshooters and the FBI says the motive behind the gunman's attempt on the life of Donald Trump remains a mystery.

Officials said Crooks was able to slip onto a rooftop location 150 yards from the stage where Trump was speaking in Butler, Pennsylvania.

Police said he then began firing an AR-15-style semiautomatic rifle, purchased by his father.

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The bullets killed a 50-year-old man, critically wounded two other spectators, and struck Trump's ear.

The officials said "a suspicious device" was found in the suspect's vehicle, which was inspected by bomb technicians and rendered safe.

A resident of Bethel Park, about an hour away from where the shooting occurred, Crooks was a registered Republican who would have been eligible to cast his first presidential vote in the Nov. 5 election.

Public records also showed that as a 17-year-old Crooks made a $15 donation to a Democratic Party cause.

Crooks was employed as a dietary aide at a nursing home at the time of the shooting.

In a statement, the nursing home administrator said, "we are shocked and saddened to learn of his involvement as Thomas Matthew Crooks performed his job without concern and his background check was clean."

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Two years ago, Crooks graduated from the local high school. One classmate, who asked not to be identified, said he showed no particular interest in politics, and his interests centered on building computers and playing games.

The classmate told Reuters, "nothing crazy ever came up in any conversation."

A retired high school counselor recalled Crooks had always been "quiet as a church mouse," "respectful" and kept to himself, although he did have a few friends.

The counsellor said Crooks was content to occasionally eat lunch by himself in the school cafeteria, and that he couldn't recall Crooks ever being disciplined in school.