STORY: A day after gunshots rang out at a Trump rally in Pennsylvania, investigators are now digging into the past of this man - Thomas Matthew Crooks - as the suspect in what police call an attempted assassination.

It's unclear how the 20-year-old managed to get close enough to shoot at Trump from a nearby rooftop with an AR-15-style rifle.

The FBI says he acted alone.

On Sunday, the police presence was heavy in Bethel Park where Crooks is from - about an hour from the rally site.

This video shows him at his high school graduation.

Jason Kohler, a classmate, described him as a 'loner'.

KHOHLER: "I mean, he would sit alone at lunch. I mean, he was just an outcast and you know how kids are nowadays." // "It's honestly kind of sad, like, I don't want to say this is what provoked it but you never know."

Crooks was shot and killed by the Secret Service.

One attendee also died - a 50-year-old man that Pennsylvania governor Josh Shapiro described as a 'hero'.

SHAPIRO: "We lost a fellow Pennsylvanian last night, Corey Comperatore."// "I asked Corey's wife if it would be okay for me to share that we spoke. She said yes. She also asked that I share with all of you that Corey died a hero, that Corey dove on his family to protect them last night at this rally."

Two other people were wounded in the shooting - but are in stable condition.

Leaders on all sides of the political spectrum condemned the shooting.

Democratic President Joe Biden on Sunday reiterated that there was no place in America for violence.

BIDEN: "An assassination attempt is contrary to everything we stand for as a nation. Everything."

Investigators say they still don't have a motive for the attack.

PALMADESSA: "I'm at a Trump rally and someone just started shooting. What's just happened?"

Johnny Palmadessa - a democratic strategist - was at the rally on Saturday to get in touch with Trump supporters... when shots rang out.

"I watched mothers shield their children from bullets. I watched husbands jump on their wives. I watched elderly people struggle to leave fast enough. Some of them practically gave up and decided, 'I'm just going to wait here because I can't get out right now'. That's not okay."

Trump is now due to receive his party's formal nomination at the Republican National Convention - which kicks off in Milwaukee, Wisconsin on Monday.

On Sunday the Secret Service said it was confident in its security plans for the event and was not anticipating any changes because of this shooting.