President Joe Biden said he had a "short but good conversation" with former President Donald Trump on Saturday night in the wake of the shooting.

"I am sincerely grateful that he’s doing well and recovering," Biden said in an address to the nation on Sunday.

No motive is known, Biden said, and he urged the public to not make assumptions.

Biden stressed that he has called on the FBI to conduct a "thorough and swift" investigation, and he said "investigators will have every resource they need to get this done."

Biden said he’s directed the head of the Secret Service to review all security measures for the Republican National Convention, which begins on Monday in Milwaukee.

"We'll share the results of that independent review with the American people," Biden said.

He added, "I've been consistent in my direction of the Secret Service to provide [Trump] with every resource, capability and protective measure necessary to ensure his continued safety."

Biden extended his "deepest condolences" to the family of the rallygoer who was killed and said he's "praying for a full recovery of those who were injured."

Biden said he will speak from the Oval Office at 8 p.m. ET on Sunday.

Former President Donald Trump said he will head to Milwaukee for the Republican National Convention on Sunday afternoon.

Trump wrote on social media that he was going to delay the trip by two days, "but have just decided that I cannot allow a 'shooter,' or potential assassin, to force change to scheduling, or anything else."

-ABC News’ Kesley Walsh

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.