The firearm recovered at the scene of the Trump assassination attempt was legally purchased by the suspect's father, an urgent trace conducted by ATF found, according to multiple law enforcement sources.

The shooter was identified by the FBI as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania.

The suspect had no apparent military affiliation.

Authorities found items of concern inside the suspect's car, which was parked outside the Trump rally, law enforcement sources told ABC News. Bomb technicians and other experts were called to investigate, but it is unclear if the items were suspicious or potentially harmful, the sources said.

A local sheriff has confirmed to ABC News that an officer tried to stop the suspect moments before shots were fired.

Josh Einiger has new information from Butler, Pennsylvania about an officer who tried to stop the suspect.

The roof where the suspected gunman fired shots at former President Donald Trump had been surveyed during security preparations for the event, a law enforcement source told ABC News.

The roof was empty during those security preparations, the source said.

As reports of suspicious activity came in, a search was initiated, the source said. It was unclear how the suspect climbed onto the roof to quickly get off shots before a Secret Service counter-sniper located and killed him, the source said.

Officials continue on Sunday to investigate whether the dead suspect was connected to domestic extremists, the source said. The early investigation turned up no ties to international terrorism, the source said.

The Bethel Park School District confirmed on Sunday that the suspected gunman was a graduate of Bethel Park High School and graduated with the Class of 2022.

"The school district wishes to express its sincere wishes for a speedy and full recovery for Mr. Trump and for those in attendance at the Saturday event who may have been physically harmed or emotionally impacted by these tragic events. We offer special condolences to the family of at least one attendee who was killed. Our thoughts and prayers are with you at this difficult time," the district said in a statement.

-ABC News' Sasha Pezenik