BUTLER, Pa. (AP) — Former President Donald Trump called for unity and resilience Sunday after an attempt on his life injected fresh uncertainty into an already tumultuous presidential campaign, while President Joe Biden implored Americans to "cool it down" in the final stretch and "resolve our differences at the ballot box."

The opponents' statements followed an attack that shook the firmament of the American political system, causing at least a temporary detente in a heated presidential campaign expected to resume again in earnest amid the pageantry of the upcoming Republican National Convention.

A full day after the shooting, the gunman's motive remained a mystery, with investigators saying they believe he acted alone before he was fatally shot by Secret Service agents. Biden ordered an independent security review of the attack, which killed a bystander, critically wounded two others and prompted questions about how a gunman was able to open fire from a rooftop near a Pennsylvania campaign rally. The FBI was investigating the shooting as a potential act of domestic terrorism.

Trump, the presumptive 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. He arrived in Milwaukee on Sunday evening for the convention, which begins Monday. He told the Washington Examiner that he had rewritten his speech for the event to focus more on national unity than on the policies of Biden.

In a post Sunday on his social media site, 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."

In a prime-time address, Biden urged the public to recommit to civil debate. "There is no place in America for this kind of violence — for any violence. Ever. Period. No exception," he said. "We can't allow this violence to be normalized."

Trump on Saturday night spoke briefly with Biden, whose Sunday night speech marked his third time addressing the shooting.

The rallygoer who was killed was Corey Comperatore, a former fire chief from the area, according to Pennsylvania Gov. Josh Shapiro, who said Comperatore "died a hero."

"His wife shared with me that he dove on his family to protect them," Shapiro said. The two wounded bystanders were listed in stable condition.

FBI investigates shooting as possible domestic terrorism

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

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 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 got so close in the first place. Kevin Rojek, the agent in charge of the FBI's Pittsburgh field office, said "it is surprising" 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, officials said. The 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 combed through his social media accounts but found no immediate threatening writing or posts, or communications indicating an ideological motive. His family was cooperating. Relatives didn't return messages seeking comment from AP.

Crooks' political leanings weren't clear. Records show him 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 Biden was sworn in.

The absence of a clear ideological motive added to deepening questions about the shooting.

Biden urged Americans to stay patient. "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 election.

FBI Deputy Director Paul Abbate said agents have seen increasingly violent rhetoric online since the rally, along with people posing online as the dead shooter. He said the FBI was focused on the upcoming political conventions in Milwaukee and Chicago.

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

Biden said the two men had a "short but good" conversation Saturday night. Biden returned to Washington from his Delaware beach home where he met with leaders in the Situation Room about the attack.

Many Republicans blamed the violence on Biden and his allies, arguing that sustained attacks on Trump as a threat to democracy have created a toxic environment.

It's unclear whether Biden will be forced to recalibrate a campaign 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 to regain the White House as a third-party candidate.

A rally disrupted by gunfire

Trump was showing off a chart of border crossing numbers when the gunfire began after 6:10 p.m. Saturday.

As the first pop rang out, Trump said "oh," raised his hand to his right ear and looked at it, then quickly crouched to the ground. People in the stands behind him also crouched as screams rang out.

Someone could be heard near the microphone saying, "Get down, get down, get down, get down!" as agents rushed to the stage. They piled atop Trump to shield him as other agents took up positions on stage to search for the threat.

Afterward, voices were heard saying, "Shooter's down" several times, before someone asked, "Are we good to move?" and "Are we clear?" Then someone ordered, "Let's move."

Trump got to his feet moments later and could be seen reaching with his right hand toward his face, streaked with blood. He then pumped his fist in the air and appeared to mouth the word "fight" twice, prompting cheers and chants of "USA. USA. USA."

His motorcade left moments later. Video showed Trump turning back to the crowd and raising a fist right before he was put into a vehicle.

Witnesses heard gunshots and ducked for cover

When the firing began, "everybody went to their knees or their prone position," said Dave McCormick, the Republican candidate for U.S. Senate in Pennsylvania, who was sitting to Trump's right on stage.

As he saw Trump raise his fist, McCormick said, he looked over his shoulder and noticed someone had been hit in the bleachers behind the stage.

Eventually, first responders were able to carry the wounded person out, McCormick said.

Reporters heard five or six shots, and many ducked for cover, hiding under tables. After the first two or three bangs, people in the crowd looked startled but not panicked. An AP reporter described the noise as sounding like firecrackers at first, or perhaps a car backfiring.

When it was clear the situation was contained and Trump wouldn't return to speak, attendees started filing out. Police soon told everyone to leave, and Secret Service agents described the site as "a live crime scene."

Republican Rep. Mike Kelly, who represents the area where the shooting occurred, attended with his wife and grandchildren and was just behind Trump when he was wounded. Kelly said he was "in a state of bewilderment of how and what has happened to the United States of America."

"I just wish people — tone it down," he said. "Quit trying to find, to blame somebody. The blame lies somewhere in the psyche of America."

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Colvin, Balsamo and Price reported from New York. Long and Tucker reported from Washington. Associated Press writers Will Weissert, Michael Biesecker, Alanna Durkin Richer, Lisa Mascaro and Tara Copp in Washington, and Marc Levy in Harrisburg, Pennsylvania, contributed to this report.