During a press briefing Sunday, U.S. Secret Service officials addressed the assassination attempt against former President Donald Trump and the upcoming Republican National Convention following Saturday's shooting.

"This is a national security event and that designation is the highest level that the federal government can determine," RNC coordinator Audrey Gibson said.

"We are confident in the security plans in place for this event and we’re ready to go, it's been an 18-month process," Gibson continued, adding, "We've worked together to develop operational security plans for any and all access of security."

Officials explained that the current Secret Service inner perimeter plan is the "largest inner perimeter that has been part of the convention" and that plans did not change following Saturday's shooting.

Officials said there are no known articulated threats against the RNC.

The man suspected of shooting Donald Trump was employed as a dietary aide at the Bethel Park Skilled Nursing and Rehabilitation Center, according to administrator Marcie Grimm.

Thomas Matthew Crooks "performed his job without concern" and passed a background check, according to Grimm.

Bethel Park Skilled Nursing and Rehabilitation Center said it's cooperating with law enforcement in their investigation.

-ABC News’ Laura Romero and Peter Charalambous

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