FBI officials have identified 20-year-old Thomas Matthew C. as the suspected gunman to have opened fire into Trump's rally in Pennsylvania.
A former classmate of Thomas told NBC News that "he was bullied so much" in high school.
The classmate said the suspected shooter was a loner, adding that he could not say what provoked Thomas to shoot into the rally.
Investigators have said the suspected shooter acted alone. Authorities "have not identified an ideology associated" with the suspected shooter, FBI Pittsburgh Special Agent in Charge Kevin Rojek told reporters on a call on Sunday.
Thomas had no known history of mental illness, according to the FBI. His family is cooperating with investigators.
The 20-year-old was a registered Republican, according to state voter records. He also donated to a Democratic political action committee when he was 17.
He was working as a dietary aide at a nursing home. The Bethel Park Skilled Nursing and Rehabilitation Center said he "performed his job without concern and his background check was clean."
FBI investigating shooting as act of 'potential' domestic terrorism
The FBI said that it was investigating a shooting at a campaign rally in Pennsylvania as an attempted assassination and a potential act of domestic terrorism.
Former President Donald Trump, who was campaigning for the upcoming presidential election in November, was hit in the ear at the rally.
One person was killed and two were injured. The shooter was fatally shot by Secret Service agents.
The FBI said that it had found a "suspicious device" which was being examined by bomb technicians after it was found in the gunman's car.
FBI agent Kevin Rojek told reporters that the gun used in the shooting was an AR-style semi-automatic rifle which was purchased legally. Investigators believed it had been acquired by the shooter's father.
The FBI said that they had not identified an ideology associated with the shooter.