President Joe Biden is rescheduling his Monday trip to Austin, Texas, where he was scheduled to deliver the keynote address at a commemoration for the 60th anniversary of the Civil Rights Act, according to the White House.

This is Biden’s first rescheduled event in the wake of the assassination attempt on former President Donald Trump. It also follows Vice President Kamala Harris postponing her Tuesday campaign stop in Florida.

Biden is expected to deliver remarks to the nation at 1:30 p.m. ET Sunday.

-ABC News' Justin Gomez

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 AR-15-style assault rifle was purchased by the suspect’s father in 2013, according to a source briefed on the investigation.

The shooter -- identified by the FBI as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania -- had no military affiliation, according to the Pentagon.

There were what "appeared" to be explosives in the suspect's car, which was parked outside the Trump rally, two sources told ABC News. One source said they looked like grenades, but the sources said they were awaiting forensic results about whether the items are actually explosives.

Former classmates of the suspected shooter, 20-year-old Thomas Matthew Crooks, of Bethel Park, Pennsylvania, described him to ABC News as a student who was "quiet" and did not have many friends.

"He was somebody who came across as lonely a lot," said Jameson Myers, who said he attended both elementary and high school with Crooks.

Myers described him as "more socially reserved" and someone who "didn't have a ton of friends."

But he said he never heard Crooks discuss politics or, specifically, Donald Trump.

"I was friends with him. He never acted, like, by any means, a political revolutionary," Myers said. "Was a very nice, even sweet guy, from what I knew."

Another classmate, Zoe Materkowski, said he was "just a quiet kid."

Added Jason Kohler, a classmate: "He was a loner. He would sit alone at lunch."

Another classmate described him as "smart" and "kind."

-ABC News’ Olivia Rubin and Peter Charalambous

The victim killed at the rally, Corey Comperatore, died a hero while protecting his family, Pennsylvania Gov. Josh Shapiro said at a news conference Sunday.

The governor said Comperatore’s wife told him that Comperatore "dove on his family."

Comperatore was a firefighter and dad of two daughters.

Shapiro said he also spoke to the family of one of the two victims critically injured in the shooting.