Gunman pointed rifle at local officer before firing at Trump during rally, sources tell AP

BUTLER, Pa. (AP) — Former President Donald Trump called for unity and resilience Sunday after an attempt on his life added fresh uncertainty to an already tumultuous presidential campaign and raised sharp questions about how a gunman was able to open fire from a rooftop near a Pennsylvania campaign rally.

WATCH: Rally in Butler, Pennsylvania cut short after shooter opens fire on Trump

A full day after the shooting, the gunman’s motive was still a mystery, and investigators said they believe he acted alone. President Joe Biden ordered an independent security review of the attack, which left one bystander dead and two others critically wounded. The FBI was investigating the shooting as a potential act of domestic terrorism.

The attack shook the firmament of the American political system, causing a reassessment and a detente — at least temporarily — of the heated 2024 presidential campaign that has grown increasingly vicious.

Trump, the Republican presidential nominee, said the upper part of his right ear was pierced by a bullet. His aides said he was in “great spirits” and doing well, and he headed to Milwaukee to the Republican National Convention that begins Monday.

“I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin,” he wrote on his social media site. “Much bleeding took place.”

In a subsequent social post Sunday, Trump said: “In this moment, it is more important than ever that we stand United, and show our True Character as Americans, remaining Strong and Determined, and not allowing Evil to Win.”

WATCH: What happened during and after the attempted assassination of Donald Trump

The rallygoer who was killed was identified as Corey Comperatore, a former fire chief from the area, according to Pennsylvania Gov. Josh Shapiro, who added that Comperatore “died a hero.”

“His wife shared with me that he dove on his family to protect them,” Shapiro said. He declined to discuss the condition of two others who were wounded.

Biden, meanwhile, spoke briefly with Trump and was to address the nation Sunday evening. The president said the country would continue to debate and disagree, but stressed: “We must unite as one nation to demonstrate who we are.”

FBI investigates shooting as possible domestic terrorism

The FBI identified the gunman as Thomas Matthew Crooks, 20, of Bethel Park, Pennsylvania, which is about 50 miles (80 kilometers) from the scene of the shooting.

READ MORE: The FBI named a suspect in the Trump assassination attempt. Here’s what we know

The gunman had his father’s AR-style rifle and was perched atop a nearby roof when some rallygoers pointed him out to local law enforcement, said two law enforcement officials who spoke to The Associated Press on condition of anonymity to discuss an ongoing criminal probe.

A local law enforcement officer climbed to the roof and found Crooks, who pointed the rifle at the officer. The officer then retreated down the ladder, and the gunman quickly fired toward Trump, the officials said. That’s when U.S. Secret Service gunmen shot him, the officials said.

Questions abounded about how the gunman could have gotten so close in the first place. Kevin Rojek, the agent in charge of the FBI’s Pittsburgh field office, said “it is surprising” that the gunman was able to open fire on the stage before the Secret Service killed him.

Bomb-making materials were found inside both Crooks’ vehicle and at his home. FBI described the devices as “rudimentary.”

His motive remained unclear. Crooks wasn’t on the FBI’s radar, and he was believed to have acted alone. Investigators are combing through his social media feeds and his home, but so far have not found any threatening writing or posts. His family is cooperating.

Crooks’ political leanings were also not clear. Records show Crooks was registered as a Republican voter in Pennsylvania, but federal campaign finance reports also show he gave $15 to a progressive political action committee on Jan. 20, 2021, the day President Joe Biden was sworn in as president.

The absence of a clear ideological motive added to the deepening questions about the shooting, denying the public any swift or tidy conclusions about the shocking crime.

Biden urged Americans to stay patient. “I urge everyone — everyone, please, don’t make assumptions about his motives or his affiliations,” he said.

Most serious assassination attempt since 1981

The attack was the most serious attempt to kill a president or presidential candidate since Ronald Reagan was shot in 1981 . It drew new attention to concerns about political violence in a deeply polarized U.S. less than four months before the presidential election.

FBI Deputy Director Paul Abbate said agents are seeing increasingly violent rhetoric online since the rally and have seen people posing online as the shooter, who is dead.

WATCH: Why political violence and violent threats are on the rise in the United States

“We’re also focused on the continuing efforts — which were already substantial, given that they’re national security special events — to conventions in Chicago, in Milwaukee,” he said.

Biden on Sunday ordered a security review of operations for the Republican National Convention, which is proceeding as planned. The Secret Service said they were “confident” in the security plan for the RNC, and no additional changes were planned.

Trump flew to New Jersey after visiting a local Pennsylvania hospital, landing shortly after midnight in Newark. Video posted by an aide showed the former president leaving his private jet flanked by Secret Service agents and heavily armed members of the agency’s counterassault team, an unusually visible show of force by his protective detail.

Biden, who is running against Trump, said the two men had a “short but good” conversation Saturday night. Biden cut short a weekend at his beach home in Rehobeth Beach, Delaware, to return to Washington, where he said during an address Sunday afternoon that “there is no place in America for this kind of violence.”

“We cannot allow this to happen,” Biden said.

Yet many Republicans quickly blamed the violence on Biden and his allies, arguing that sustained attacks on Trump as a threat to democracy have created a toxic environment. They pointed in particular to a comment Biden made to donors on July 8, saying “it’s time to put Trump in the bullseye.”

A person familiar with those remarks said the president was trying to make the point that Trump had gotten away with a light public schedule after last month’s debate while the president himself faced intense scrutiny from many even within his own party. The person spoke on the condition of anonymity to more freely discuss private conversations.

In the donor call, Biden said: “I have one job and that’s to beat Donald Trump … I’m absolutely certain I’m the best person to be able to do that.”

Biden continued then: “So, we’re done talking about the debate. It’s time to put Trump in the bullseye. He’s gotten away with doing nothing for the last 10 days except ride around in his golf cart, bragging about scores he didn’t score … Anyway I won’t get into his golf game.”

But it’s not clear yet whether Biden will be forced to recalibrate a campaign that’s largely focused on Trump as a threat to democracy. It is a situation the U.S. has not seen since Teddy Roosevelt was shot a month before the election in 1912 while campaigning trying to regain the White House as a third-party candidate.