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(NewsNation) — President Joe Biden is calling for an independent investigation into the attempted assassination of Donald Trump after a shooter opened fire on the former president’s rally in Pennsylvania Saturday.

Early Sunday morning, the FBI identified the attempted assassin as Thomas Matthew Crooks, a 20-year-old from Bethel Park, Pennsylvania. He was shot and killed by the Secret Service moments after shots first rang out.

Sources tell the Associated Press that a local law enforcement officer encountered the gunman moments before he fired toward former President Donald Trump during the rally. The officials spoke to AP on condition of anonymity to discuss an ongoing investigation.

They said not long before shots rang out, rallygoers noticed a man climbing to the roof of a nearby building and warned local police. When an officer climbed to the roof, Crooks pointed his rifle at the officer.

The sources say the officer retreated down the ladder and Crooks quickly took a shot toward Trump, and that’s when the U.S. Secret Service counter snipers shot him. Trump was injured in the shooting but is “fine.

Trump shooting a ‘potential’ domestic terrorist act

A top official at the FBI said on Sunday the agency is investigating the shooting that took place at Trump’s campaign rally as an assassination attempt in addition to a potential domestic terrorism act.

“At this point [in] the investigation, it appears that he was a lone actor, but we still have more investigation to go,” explained Robert Wells, the executive assistant director of the FBI’s National Security Branch, referring to the suspected shooter.

“We are investigating this as an assassination attempt,” but also as a “potential domestic terrorism act,” he explained. “So our counterterrorism division and our criminal divisions are working jointly together to determine the motive.”

Wells also said they had received over 2,600 tips regarding the attempted assassination against Trump in Pennsylvania.

“We also have a tip line that has been set up, and we’ve received over 2,600 tips,” Robert Wells, the executive assistant director of the FBI’s National Security Branch, told reporters.

“Every single tip that we receive, we will be investigating and running it down as quickly as we possibly can,” he added.

FBI confirms the weapon was AR-style rifle

Senior FBI officials confirmed with NewsNation’s Evan Lambert that Crooks had “no indication of mental health issues,” and that no threats toward the president or event have been found.

Officials also said a legally-obtained AR-style 556 rifle was located “at scene adjacent to shooter.” It was purchased by Crooks’ father, but how the 20-year-old accessed it is unknown at this time. The shooter’s family is cooperating with law enforcement.

The scene is still being processed, and DNA, weapon and fingerprint testing continues.

Kevin Rojak, FBI special agent in charge, detailed that a search of the shooter’s car turned up “a suspicious device.” Bomb technicians inspected it and neutralized it.

“This caused a significant delay in our processing of the scene for officer safety reasons,” he said.

The device is also on its way to the Quantico lab for analysis.

The suspected shooter, Thomas Matthew Crooks, was a dietary aide at the Bethel Park Skilled Nursing and Rehabilitation Center, the organization said in a statement on Sunday, adding that they were “shocked and saddened” to hear he was involved.

Trump rally shooting victims identified

Besides Crooks, one other person was killed and two others were critically injured in the attack. Firefighter Corey Comperatore, 50, was identified as the person who died in the attack. He was shielding his family from the bullets before he was struck, according to his sister, Dawn, Pennsylvania Gov. Josh Shapiro said at a news conference Sunday.

Amid an attempt to assassinate Trump, David Dutch, 57, and James Copenhaver, 74, were also injured, according to police. Both men were listed as in stable condition.

Trump’s campaign raised more than $2 million through a GoFundMe campaign this weekend for the victims. By Sunday afternoon, just before 3 p.m., the Trump campaign more than doubled its $1 million goal, reaching $2.034 million in donations. The total number of donations reached 27,900, according to the website, and the top donation was listed as $50,000.

Biden, Trump respond to assassination attempt

“There is no place in America for this kind of violence or any violence for that matter. An assassination attempt is contrary to everything we stand for as a nation, everything. It’s not who we are as a nation, it’s not America and we cannot allow this to happen,” Biden said Sunday.

After the shooting, Biden quickly denounced the attack and talked to Trump in its aftermath. His campaign team, meanwhile, is grappling with how to manage the political implications of an attack on the man Biden hopes to defeat in the November election.

In the wake of the violence, both Biden and Trump called for unity and resilience as shocked leaders across the political divide reacted to the shooting.

“Unity is the most elusive goal of all, but nothing is more important than that right now,” Biden said Sunday. “We’ll debate and we’ll disagree, that’s not going to change, but we’re going to not lose sight of the fact of who we are as Americans.”

Biden said he had a good talk with former President Trump and that he has directed an independent review of security at the Pennsylvania rally where the shooting took place.

Biden and Trump spoke the night prior and the president said, “I’m sincerely grateful that he’s doing well and recovering. We had a short but good conversation.”

Biden addressed the nation once more on Sunday evening, speaking from the Oval Office at 8 p.m. E.T. touching on “the need for every American to come together” to end political violence in the U.S.

Biden spoke for about five minutes from the Oval Office and noted that the Republican National Convention was opening in Milwaukee on Monday, while he himself would be traveling the country to campaign for reelection.

He said passions would run high on both sides and that the stakes of the election were enormous.

But the president added, “it’s time to cool it down” and noted not just the weekend attack on Trump but also the possibility of election-year violence on multiple fronts.

Meanwhile, Trump told The Washington Examiner that he has rewritten the speech he was set to deliver at the RNC. “The speech I was going to give on Thursday was going to be a humdinger,” he told the news outlet in an article posted Sunday evening.

In the interview, the presumptive Republican presidential nominee says he will now call for a new effort at national unity, noting that people from different political views have called him.

Secret Service give RNC security update

Trump says he’ll keep his plan to travel to Milwaukee on Sunday afternoon ahead of the Republican National Convention.

Trump said on his Truth Social platform that he was going to delay his trip after Saturday’s apparent assassination attempt, but decided he cannot “allow a ‘shooter,’ or potential assassin, to force change to scheduling, or anything else.”

The U.S. Secret Service said it is not making any changes to its security plans for this week’s Republican National Convention in Milwaukee where Trump is slated to be named officially the party’s presidential candidate.

The agency is not planning to expand the inner perimeter for the RNC in Milwaukee and will not ban firearms in the outer perimeter, officials said at a press conference Sunday.

“Our state law allows for the particular right to carry a firearm. We, as a city, cannot legislate out of that,” Milwaukee Police Chief Jeffrey Norman said at the press conference, also noting that the inner perimeter is among the largest ever at a presidential party convention.

NewsNation partner The Hill contributed to this report.