A 20-year-old Pennsylvania man, Thomas Matthew Crooks, has been identified as the suspected shooter who tried to assassinate former president Donald Trump at a campaign rally this weekend.

Secret Service snipers killed Crooks on Saturday after he fired multiple shots at the rally in Butler, Pennsylvania, from a nearby rooftop.

As authorities seek to understand his motivations, here is what we know of the alleged gunman:

'Quiet'

Crooks' former schoolmates described him as a "quiet" student who often came across as "lonely," ABC News reported.

He seemed "socially reserved," but a former schoolmate did not recall hearing him discuss politics or Trump.

Trump, 78, was hit in the ear at the rally. A bystander was killed and two spectators critically injured.

Jason Kohler, who said he attended school with Crooks, recalled that the alleged shooter had often been bullied.

"He was quiet but he was just bullied. He was bullied so much," Kohler told reporters.

He said Crooks had been made fun of for the way he dressed, noting that he would sometimes wear hunting outfits.

Dan Grzybek, a county council representative in the area where Crooks grew up, said the neighborhood was "pretty firmly middle class, maybe upper-middle class," according to The New York Times.

The Times added that Crooks had been working at a nursing home, and the organization was shocked to hear of his involvement in the shooting.

No military ties

Following the incident, investigators found a "suspicious device" in Crooks' car, which was inspected by bomb technicians and is being analyzed.

They noted that suspected explosive devices have been collected as evidence.

Authorities are now in the process of searching Crooks' phone.

Kevin Rojek, special agent in charge of the FBI's Pittsburgh field office, told reporters Sunday that the gun used in the shooting was an AR-style semi-automatic rifle which was purchased legally.

Investigators believe the weapon was purchased by Crooks' father.

Rojek added that there were no indications so far of any mental health issues.

Crooks also did not have military affiliations, according to Pentagon spokesman Major General Pat Ryder.

The FBI told reporters that they are investigating the incident as an assassination attempt and also a potential domestic terrorism act.

Acted alone

The shooter was believed to be working alone, according to the FBI, and officials said they have not identified an ideology associated with him.

His political leanings were not instantly clear.

News reports indicated he was a registered Republican, but also noted that he previously gave money to a Democratic-aligned progressive political action committee.

While social media platform Discordetha said it had identified an account that appeared linked to the suspect, this was "rarely utilized."

A spokesperson added that "we have found no evidence that it was used to plan this incident, promote violence, or discuss his political views."

The suspect's father Matthew Crooks on Saturday told CNN that he was trying to establish "what the hell is going on" before speaking about his son.