Authorities "have not identified an ideology associated" with the suspected shooter, Thomas Crooks, FBI Pittsburgh Special Agent in Charge Kevin Rojek told reporters on a call on Sunday.

"We're looking into his background, his day-to-day activities, any writings and social media posts that might help us identify what led to this shooting. And we have not seen anything threatening," Rojek said.

Crooks had no known mental illness history, he said.

Crooks’ family is cooperating with federal authorities, officials said.

Officials told reporters they don’t have any insights on what the suspect's actions were prior to opening fire at the shooting site, but said it's something they are investigating.

Robert Wells, the FBI's assistant director of the Counterterrorism Division, told reporters, "We are investigating this as an assassination attempt, but also looking at as a potential domestic terrorism act."

The FBI has received more than 2,000 tips, according to Rojek.

Crooks’ gun and phone will be transported to the FBI lab in Quantico, Virginia, officials said.

Rojek also confirmed there was a suspicious device that was found in the suspect’s car and said the FBI is still analyzing what it is. Another device was found in the suspect’s home, according to the FBI. Rojek described both the devices as rudimentary.

The FBI is seeing an increase of violent threats online in the aftermath of the assassination attempt, FBI Deputy Director Paul Abbate said.

"We've seen individuals go online and attempt to mimic or pose as the shooter who obviously is now deceased," Abbate said. "We're also focused on the continuing efforts, which were already substantial, given that there are national special security events on the upcoming conventions in Chicago, in Milwaukee, to work with Secret Service in the lead to play our part in protecting the people and the facilities and events there, as well."

FBI Director Christopher Wray called the assassination attempt an "attack on our democracy."

"An attempt to assassinate a presidential candidate can only be described as absolutely despicable and will not be tolerated in this country," Wray told reporters.

-ABC News’ Pierre Thomas, Aaron Katersky, Jack Date and Luke Barr

The man suspected of shooting Donald Trump previously sought to join his high school’s rifle club, but was rejected and asked not to return, two students who were members of the team at the time told ABC News.

The suspect, Thomas Crooks, attended what was described as "preseason" for the rifle club, which is held before tryouts, but he did not make it to the tryouts, the two team members said. Both students said Crooks was a "bad shot," with one adding that he wasn't the right "fit."

"He didn't just not make the team -- he was asked not to come back because how bad of a shot he was. It was considered like, dangerous," classmate Jameson Myers told ABC News.

"On the first day of preseason, he basically couldn't even hit the target," Myers said.

The team’s coach declined to comment and referred questions to the school district. A spokesman for the school district did not immediately respond to a request for comment.

-ABC News' Olivia Rubin

The two people injured in Saturday’s shooting have been upgraded from critical condition to stable condition, according to the Pennsylvania State Police.

Police identified 57-year-old David Dutch, of New Kensington, Pennsylvania, and 74-year-old James Copenhaver, of Moon Township, Pennsylvania, as the injured shooting victims.

Killed in the shooting was Corey Comperatore, 50, of Sarver, Pennsylvania.

Pennsylvania Gov. Josh Shapiro has ordered flags to be flown at half-staff in honor of Comperatore, a firefighter who leaves behind a wife and two daughters.

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.