Mr. Trump was escorted off the stage bleeding from around his ear, and taken to a hospital. He later said on social media that a bullet had pierced the upper part of his right ear.

President Biden gave a brief televised statement after the incident, condemning the violence as “sick.” The White House later said the president had spoken to Mr. Trump.

Here’s what to know:

• Two videos posted on social media and verified by The New York Times appeared to show the suspected shooter lying motionless on the roof of a small building roughly 400 feet north of the stage where former President Donald J. Trump was speaking. Here’s a map of the site.

• Mr. Trump had been showing supporters a chart of numbers about border crossings just minutes into his speech when shots rang out in two bursts. Read firsthand accounts from our reporter and photographer.

• A spokesman for the Secret Service said that the suspected shooter fired “multiple shots toward the stage” and was on “an elevated position” outside the rally venue. The Secret Service and other law enforcement agencies have not yet publicly confirmed that Mr. Trump was shot in the ear, saying only that shots were fired and that the former president was “safe.”

• Two law enforcement officials said that authorities recovered an AR-15-type semiautomatic rifle from a deceased man they believe was the gunman. People who attend Mr. Trump’s campaign rallies are subject to security screenings, and their belongings are typically searched for weapons.

• After the shooting, Mr. Biden expressed gratitude that Mr. Trump had been swiftly evacuated and said “there’s no place in America for this kind of violence.” Mr. Biden’s campaign said in a statement that it would pause “all outbound communications” and was working to “pull down our television ads as quickly as possible.” Read more about the president’s response.

• Republicans and Democrats were quick to condemn what they viewed as an apparent act of political violence. Read more about the reaction.

• The Republican National Convention, where Mr. Trump will be formally nominated as his party’s presidential candidate, will proceed as planned in Milwaukee starting on Monday, according to a statement from the Trump campaign and R.N.C. officials.

Reporting was contributed by Nicholas Nehamas, Glenn Thrush, Campbell Robertson, Peter Baker and Katie Rogers.

“Then the president went down, and I didn’t know if he took a body shot, a head shot or what,” Mr. Miller said in an interview hours after the shooting.

Mr. Miller, who was about 30 feet away from Mr. Trump, said the people around him lowered their heads and screamed. He stood near a railing, looked up and saw Mr. Trump swarmed by Secret Service agents who were shielding him.

Mr. Miller said he surveyed his surroundings, trying to determine where the bullets had come from. He saw people ducking their heads, and others scanning in circles, similarly flummoxed by what was happening.

Some attendees nearby screamed, while others cried. Security personnel carrying firearms paced throughout the crowd, clearly “on edge,” Mr. Miller said.

A few rows behind Mr. Miller, he said, was a person bleeding profusely, whose white clothes were reddening as people gathered around to help.

Mr. Miller then looked back at Mr. Trump, who was back on his feet, his face partly smeared with blood, with a wall of security personnel around him. The crowd chanted “U.S.A.! U.S.A.!” as he was rushed off the stage.

Soon, security officers directed people to exit outside.

“It was an atmosphere of shock,” Mr. Miller said.

The latest such episode was in 1972, when Gov. George C. Wallace of Alabama was shot while campaigning at a shopping center outside Washington, D.C. Mr. Wallace was partly paralyzed in the shooting and used a wheelchair until his death in 1998.

Four years earlier, Robert F. Kennedy, a senator and former U.S. attorney general, was fresh off winning California’s 1968 Democratic primary when he was fatally shot after giving a victory speech at the Ambassador Hotel in Los Angeles. His son Robert F. Kennedy Jr. is currently running for president as an independent candidate and has unsuccessfully sought Secret Service protection.

President John F. Kennedy, an older brother of Robert F. Kennedy, was fatally shot by Lee Harvey Oswald while visiting Dallas in November 1963 to shore up support for his re-election bid in 1964. His assassination shocked the nation and has spurred countless conspiracy theories over the decades.

Before Saturday, there had been at least 15 direct assaults on presidents, presidents-elect and presidential candidates, and five of them resulted in deaths, according to the Congressional Research Service.

In 1975, there were two assassination attempts on President Gerald R. Ford in less than three weeks. In the first, Lynette A. Fromme, an acolyte of Charles Manson, tried to fire a pistol at Mr. Ford as he walked from his hotel to the State Capitol in Sacramento, but the chamber had no bullet. Seventeen days later, Sara Jane Moore, who had been involved with several leftist groups, tried to shoot the president outside a hotel in San Francisco but missed when a Marine who was standing next to her knocked her arm upward as she fired.

In March 1981, about two months after his inauguration, President Ronald Reagan was shot and seriously injured outside a Washington, D.C., hotel by John W. Hinckley Jr., who had been trying to draw the attention of the actress Jodie Foster after seeing her in the movie “Taxi Driver.” Mr. Hinckley was unconditionally released in 2022.

Then there was a new sound. Pop.

A firecracker, it seemed. Mr. Trump grabbed at his ear. And then: Pop. Pop. Pop.

Suddenly, a day of big emotions — glee, unity and righteous anger — was now shattered by fear. Around the grandstand where Mr. Trump had been speaking, beneath an enormous American flag suspended between two cranes, the spectators crouched. Secret Service officers swarmed over Mr. Trump.

It was now clear what was happening. Thousands of people, from those in the bleachers to the many watching from a large grassy field, dropped to the ground almost in unison.

Just before the shots rang out, some in the crowd said, it seemed as if law enforcement snipers who were perched atop a barn had noticed movement nearby. The snipers seemed to be focusing on something off to the side of the grandstand in the direction of a building and a water tower just outside the farm show grounds.

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“I saw them with their binoculars,” said Craig Cyrus, 54, who had come over from New Castle, Pa. “Then they got their guns.”

Once the shooting broke out, he said, the snipers returned fire.

“The first thing I thought to myself was, ‘America’s under attack,’” said Corey Check, a local conservative activist and Republican committeeman. “I grabbed the hands of a couple of people I didn’t even know. We said the Lord’s Prayer.”

Some began to cry and others screamed, while law enforcement officers shouted for everyone to get down. One of the audio speakers, apparently hit by a gunshot, toppled over.

When the popping ceased just a few seconds later, and heads rose again, the grim aftermath came into view.

Sid Miller, the commissioner of the Texas Department of Agriculture, who had come to the rally to cheer on Mr. Trump, said he turned and saw a person behind him bleeding profusely, their white clothes reddening as people gathered around to help. Others saw a man directly behind Mr. Trump, who appeared to be bleeding from the head. Several at the rally said they would later see a couple of people being carried out, limp and covered in blood.

According to the Secret Service, one person who attended the rally was killed, along with the suspect, and two spectators were seriously injured. The F.B.I. identified the suspect as Thomas Matthew Crooks, 20, of Bethel Park, Pa. And the authorities recovered an AR-15-type semiautomatic rifle at the scene, according to two law enforcement officials.

In the moment, all eyes quickly turned to Mr. Trump. Some, in the few seconds that felt like an infinity, were fearing the worst.

“I couldn’t stop crying,” said Eduardo Vargas, 31, who was sitting not far behind Mr. Trump. “I thought I just saw the president get killed in front of my face.”

But Mr. Trump returned to his feet. He had a little blood on his forehead. But he seemed not to have been badly injured. He raised his fist in the air.

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The crowd cheered “USA! USA!” though the cheers were not quite as robust as they had been just a few minutes earlier. Mr. Trump was ushered into an SUV by law enforcement officers. Later, on Truth Social, Mr. Trump said that he had been “shot with a bullet that pierced the upper part of my right ear,” and Secret Service officials said he was safe.

Other officers then told everyone to leave, and so the tens of thousands who had come to see Mr. Trump, and sweated for hours for the chance, headed for the exits in a fog of shock and disorientation.

“I’m still so shaken up,” said Tiona Evans, 48, a child care provider from Pittsburgh, who had attended the rally as a birthday gift to herself.

The spectators returned to their trucks and cars, comparing notes on what they had seen and heard, some passing along ominous rumors or acknowledging that they had not seen much of anything in the chaos.

Nearly an hour after the shooting, after the grounds had emptied and the traffic had eased on the way out of town, Mr. Vargas, who was standing outside the entrance, was still rattled.

“I love Trump, I care for Trump, I fear for his life,” he said. “More than anything, I was scared that they got him.”

Eduardo Medina, Brian Conway, Michael Gold, Julian E. Barnes and Simon J. Levien contributed reporting.

Mr. Mills was using a Sony digital camera capable of capturing images at up to 30 frames per second. He took these photos with a shutter speed of 1/8,000th of a second — extremely fast by industry standards.

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The other factor is the speed of the bullet from the firearm. On Saturday law enforcement authorities recovered an AR-15-type semiautomatic rifle at the scene from a deceased white man they believe was the gunman.

“If the gunman was firing an AR-15-style rifle, the .223-caliber or 5.56-millimeter bullets they use travel at roughly 3,200 feet per second when they leave the weapon’s muzzle,’’ Mr. Harrigan said. “And with a 1/8,000th of a second shutter speed, this would allow the bullet to travel approximately four-tenths of a foot while the shutter is open.”

“Most cameras used to capture images of bullets in flight are using extremely high speed specialty cameras not normally utilized for regular photography, so catching a bullet on a side trajectory as seen in that photo would be a one in a million shot and nearly impossible to catch even if one knew the bullet was coming,” he said.

In Mr. Harrigan’s last assignment, he led the bureau’s firearms training unit and currently works as a consultant in the firearms industry.

“Given the circumstances, if that’s not showing the bullet’s path through the air, I don’t know what else it would be,” he said.

A spokesman for the Secret Service, Anthony Guglielmi, said the suspected gunman “fired multiple shots toward the stage from an elevated position outside the rally venue.” The suspect had been killed, the Secret Service said.

The audio analysis of the gunshots conducted by The Times and Robert C. Maher, a gunfire acoustics expert at Montana State University in Bozeman, indicates that two bursts of shots were fired. Both the first round of three and the second of five shots were fired approximately 330 to 390 feet from the C-SPAN microphone Mr. Trump was speaking into. That location was consistent with the location of the suspect’s body. There was no significant difference between the sound of the eight shots, which suggests that they likely came from the same firearm, Mr. Maher said.

A witness told the BBC that he saw an armed man in “in muted colors, tan-typed clothing” climbing up the roof of a building of the same compound captured in the videos. The witness, who gave his name as Greg, said he attempted to alert the police to the man’s presence, minutes before the shots were fired.

And then his arm reached toward the sky, and he began punching the air.

The crowd started to chant — “U-S-A! U-S-A!” — as the agents inched Mr. Trump toward the stairs. When they reached the top step, they paused once more, so Mr. Trump could lift his arm a little higher, and pump his fist a little faster. The crowd roared a little louder.

It’s difficult to imagine a moment that more fully epitomizes Mr. Trump’s visceral connection with his supporters, and his mastery of the modern media age.

Mr. Trump would not leave the stage without signaling to his fans that he was OK — even as some were still wailing in fear. And he did not just wave or nod, he raised his fist in defiance above his bloodied face — making an image history will not forget.

He has always been highly conscious of how he looks in big moments, practicing his Clint Eastwood squint and preparing for his mean mug-shot grimace. But there was no time to prepare for this.

This was instinct.

As the agents coaxed him onto his feet, he stammered, “Let me get my shoes on, let me get my shoes on.”

“I got you, sir, I got you, sir,” an agent replied. Mr. Trump rose, his voice uneven at first, still repeating himself: “Let me get my shoes on.”

“Hold that on your head,” an agent told him, “it’s bloody.”

“Sir, we’ve got to move to the cars,” another said.

“Let me get my shoes on,” Mr. Trump said again.

Fierce one moment, he looked drained and stricken the next.

After the agents managed to hustle him off the stage, they led him toward an idling Chevrolet Suburban. He began to clamber inside, but before the door could close, he turned back toward the crowd again. His head appeared more blood-soaked than before. He raised his fist one more time.

What did you see and hear today?

It was a very standard, typical rally. The former president was maybe an hour late. The crowd had been hot all day. Donald J. Trump arrived, waving to the crowd, just like any other rally he does.

There’s a pool of photographers, maybe four of us, who were in what is called the buffer area just a couple feet from the former president. We were all jostling around in there trying to get our normal pictures.

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All of a sudden, there was what I thought were three or four loud pops. At first I thought it was a car. The last thing I thought was it was a gun.

I kept taking pictures. He went down behind the lectern, and I thought, “Oh my God, something’s happened.”

Then all the agents started running on the stage, and they basically completely covered him. I could hear them yelling. At first someone saying, “Sir, sir, sir.”

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With that, the counter-snipers, also members of the Secret Service, whom we rarely see unless they’re on a roof or something, they come up out of nowhere and were up on the stage holding automatic rifles.

I went from one side of the stage to another to see if I could see him any better. And that’s when he got up and put his fist in the air. And I thought, “He’s alive, he’s alive.”

I could see blood on his face. I kept taking pictures. As tough as he looked in that one picture with his fist looking very defiant, the next frame I took, he looked completely drained. Very, very shocked.

As he came down the steps, the Secret Service completely covered him in a blanket of people and they walked him all the way to his SUV.

What happened in the aftermath?

I turned around and I saw people screaming and heard somebody had been shot in the crowd. They held us in Trump’s normal holding tent for probably 30 minutes.

When we came out we saw the littered field, plastic bottles, cellphones, a motorized wheelchair just abandoned.

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In your career, have you ever been in a similar situation?

I’ve always feared being in this situation. I always wondered what I’d do in this situation. I hope I get the right shot. I hope I’m not shot myself.

At first I thought right away, “Could I be shot?” It was scary.

I’ve never been in a more horrific scene. As much as I’ve covered presidents for 35 to 40 years, it’s not something I ever wanted to witness.

There were a lot of members of his staff backstage crying, I got lots of hugs all just saying, “I’m so glad we’re OK.”

I never envisioned being in a situation like this.

One spectator was killed in the shooting at Mr. Trump’s rally, and the suspected shooter was killed by law enforcement, according to the Secret Service. Mr. Trump wrote in a post on his social media platform that he had been “shot with a bullet that pierced the upper part of my right ear.” The Secret Service and other law enforcement agencies have not yet publicly confirmed that Mr. Trump was shot in the ear, saying only that shots were fired, that the former president was “safe” and that the shooter was now dead.

Investigators have released few details about the suspect, or his motive. But some politicians who have been the target of politically motivated violence, or attempted violence, offered their condolences to the former president.

Representative Nancy Pelosi of California, the former Democratic House speaker whose husband was attacked at their home in 2022 by an assailant who was looking for her, wrote in a post on X that she knew “firsthand that political violence of any kind has no place in our society.”

“I thank God that former President Trump is safe,” she added.

Gov. Gretchen Whitmer of Michigan, a Democrat who was the target of a foiled kidnapping plot, said on X that she was “horrified” to hear the news of the violence at the Trump rally.

Political violence cuts across the nation’s partisan divide, and the attack at the Trump rally comes as threats of such violence are increasing in the United States. Judges, members of Congress and local elections officials have all been besieged with threats in recent years.

Those threats often do not result in acts of violence, but some political figures have reported deeply harrowing experiences. A person who had written threatening emails to Representative Pramila Jayapal of Washington, a Democrat, showed up outside her house with a gun. Someone smashed a storm window at the home of Senator Susan Collins of Maine, a Republican.

And in 2017, a gunman opened fire on a group of Republicans practicing for a congressional baseball game. Representative Steve Scalise of Louisiana was gravely injured.

“There is never any place for political violence,” Mr. Scalise wrote on X after the shooting at the Trump rally on Saturday, before criticizing in a separate post what he called the “incendiary rhetoric” of Democrats. “We’ve seen far left lunatics act on violent rhetoric in the past,” he wrote.

The shooting at Mr. Trump’s rally “is a consequence of such significant support for political violence in our country,” Mr. Pape wrote in an email. “Indeed, significant lone wolf attacks motivated by political violence have been growing for years in the United States, against members of Congress from both parties as well as federal officials and national leaders.”

Other studies on political violence have also found small but not insignificant numbers of Americans who support the idea of using violence to advance political ideas.

In October, the Violence Prevention Research Program at the University of California, Davis, published a report that found nearly 14 percent of those surveyed strongly agreed that there would be a civil war in the United States in the next few years.

Nearly 8 percent of respondents to the study said they believed there would be a situation in the next few years where political violence would be justified and were intending to arm themselves.

They were sitting close enough to the stage that as Mr. Trump spoke, they could take photos of themselves with the former president in the background. They watched in shock as Secret Service agents rushed toward Mr. Trump.

Some of the other attendees in their section tried to flee the chaotic scene that followed, Mr. Check said, but a Secret Service agent ordered everyone to get down.

“The first thing I thought to myself was, America’s under attack,” Mr. Check said. “I grabbed the hands of a couple of people I didn’t even know. We said the Lord’s Prayer. I called my family and told them I loved them.”

When they were allowed to stand up, Mr. Rybner said he saw what appeared to be blood on a higher level of the seating section behind them. “There was a lot of blood,” he said.

Before the rally devolved into chaos, while Mr. Rybner was still waiting for Mr. Trump to arrive onstage, he said he passed the time by looking at the crowd in the section. “I didn’t see anything out of the ordinary,” he said.

Mr. Check, speaking by phone minutes after the shooting, was still struggling to process what he had just experienced.

“We chanted ‘U.S.A.’” after all this happened, he said, adding that the country will live on “despite what some maniac did to Trump.”

“We’re alive. And we will never stop. America has been here, we will always be here, chanting ‘U.S.A.’ Because we’re not done. These people will not destroy our country,” he said.

“I have been briefed on the shooting at Donald Trump’s rally in Pennsylvania,” Mr. Biden said in the statement. “I’m grateful to hear that he’s safe and doing well. I’m praying for him and his family and for all those who were at the rally.”

He continued: “There’s no place for this kind of violence in America. We must unite as one nation to condemn it.”

The top Republicans in Congress — Senator Mitch McConnell of Kentucky and Speaker Mike Johnson of Louisiana — and their Democratic counterparts — Senator Chuck Schumer of New York and Representative Hakeem Jeffries of New York — also quickly published statements denouncing the shooting.

“My thoughts and prayers are with former President Trump,” Mr. Jeffries said, adding, “America is a democracy. Political violence of any kind is never acceptable.”

Mr. McConnell said: “Tonight, all Americans are grateful that President Trump appears to be fine after a despicable attack on a peaceful rally. Violence has no place in our politics.”

Representative Steve Scalise, the No. 2 Republican in the House, said in a statement that he was “praying for President Donald Trump, adding, “There is never any place for political violence.” Mr. Scalise was shot and gravely wounded in 2017 at a practice of the Republican congressional baseball team.

Scores of other Republican and Democratic officials condemned the shooting and emphasized that political violence had no place in American democracy.

Robert F. Kennedy Jr., the independent presidential candidate whose father was murdered during his presidential campaign, also condemned the attack in a statement.

“Now is the time for every American who loves our country to step back from the division, renounce all violence and unite in prayer for President Trump and his family,” he said.

Prime Minister Benjamin Netanyahu of Israel offered prayers for Mr. Trump in what was the early morning in Israel, saying he was “shocked by the apparent attack on President Trump.” Prime Minister Keir Starmer of Britain also said in a statement that he was “appalled” by the shooting, adding, “Political violence in any form has no place in our societies, and my thoughts are with all the victims of this attack.”

Gov. Gavin Newsom, Democrat of California and a close ally of Mr. Biden and his campaign, said in a statement: “Violence has no place in our democracy. My thoughts are with President Trump and everyone impacted at the rally today.”

Senator Bob Casey, a Pennsylvania Democrat, wrote on social media that he had offered support to the state police. “Political violence is never acceptable and I am hoping former president Trump & all attendees are safe.”

Liz Cheney, a vocal Republican critic of Mr. Trump — who the former president has said should be jailed — said in a statement on social media that people should “pray for the former president and all those injured,” adding that “violence of any kind has no place in American politics.”

Two former presidents, Barack Obama and Bill Clinton, also denounced the attack in statements that evening. Mr. Clinton said he and his wife, Hillary Clinton, were “heartbroken for all those affected by the attack.” Mr. Obama called on Americans to “use this moment to recommit ourselves to civility and respect in our politics.”

Reporting was contributed by Catie Edmondson, Robert Jimison, Theodore Schleifer, Lisa Lerer Katie Glueck and Shane Goldmacher.

The former president, who was rushed off the stage with blood on his right ear and his face, added that “much bleeding took place, so I realized then what was happening.”

In his post, Mr. Trump extended his condolences to the family of the person attending his rally who was killed, and to the family of “another person that was badly injured.”

He added that he did not know anything about the shooter, who officials said had been killed.

“I want to thank The United States Secret Service, and all of Law Enforcement, for their rapid response on the shooting that just took place in Butler, Pennsylvania,” Mr. Trump wrote.

The Secret Service and other law enforcement agencies have not yet publicly confirmed that Mr. Trump was shot in the ear, saying only that shots were fired, that the former president was “safe” and that the shooter was now dead.

Disinformation experts immediately urged caution, warning people not to jump to conclusions.

“We are about to see a lot of disinformation spreading about who is behind the shooting, who executed it, and the events that led up to this moment,” Roberta Braga, the founder of the think tank Digital Democracy Institute of the Americas, wrote on X. “Watch for emotional language.”

Law enforcement officials will require time to investigate what happened at the rally. Video from the event appears to show Mr. Trump falling to the stage and bleeding from his ear before being escorted away by Secret Service agents.

Some social media accounts said that a gunman with ties to the “deep state” had opened fire and shot Mr. Trump. Without citing evidence, the accounts also blamed government agencies such as the F.B.I. and the Centers for Disease Control for targeting Mr. Trump.

Representative Mike Collins, Republican of Georgia, quickly accused President Biden of ordering an apparent shooting at Mr. Trump’s rally, but provided no proof. “Joe Biden sent the orders,” he wrote on X. Mr. Collins didn’t immediately respond to requests for comment.

Others claimed on social media that any shooter was a member of the far-left Antifa group, or was acting on behalf of transgender individuals. None of the posts provided evidence for their theories.

The county that the city sits within has been changing over time, however, becoming both more educated and more prosperous.

Broader Butler County’s population of nearly 200,000 remains about 95 percent white, according to the Census Bureau, but the nonwhite share of the population has been slowly growing. About 38 percent of adults there now have a bachelor’s degree, slightly higher than the 34 percent average nationally.

The county’s unemployment rate is well below the national level: just 2.8 percent. And per capita earnings in Butler County surpassed the state average in 2007, after being below it ever since records had begun to be kept.

Professional and business services are now the county’s largest employer. The southern part of the county is accessible to Pittsburgh, which makes it popular among commuters into the city. New housing developments and businesses have sprung up in recent years.

But Butler County retains large swaths of rural farmland and wooded forests. Gun ownership in that corner of Western Pennsylvania is pervasive, and hunting is such a major pastime there that local schools long took off the first day of deer season.

“We’re a little overwhelmed,” Bob Dandoy, Butler’s mayor, said in an interview late Saturday, after the shooting. “This is a small town.”

The area skews heavily Republican, and Trump signs dot local roadsides. Voter registration data from the local Bureau of Elections shows that just under 40,000 Democrats are registered in the country, and nearly 80,000 Republicans. About 20,000 voters are not registered as members of either party.

Trump voters outnumbered Biden voters nearly two to one in the 2020 election.

Mr. Dandoy is a Democrat, but he said the trauma of the shooting transcended parties.

“Politics aside, the people at that rally were exercising one of the most basic freedoms of our country,” he said. “An attack on their rights is an attack on all of our rights.”

The Secret Service told everyone to “Get down! Get down!” Mr. Vargas said.

“I saw half the people around me start crying,” he said. “And I started crying. I couldn’t stop crying.”

Mr. Vargas said he feared the worst.

“I thought I just saw the president get killed in front of my face,” he said.

In the chaos that followed, Mr. Vargas said he saw one man on Mr. Trump’s team who seemed to be covered in blood, though he wasn’t sure whose blood it was. He also said he saw a woman get carried away who appeared to be limp.

Mr. Vargas said a pregnant woman reached down to him and asked him if he was OK. But he remains shaken.

“I love Trump, I care for Trump, I fear for his life,” he said. “More than anything, I was scared that they got him.”

Mr. Biden said he had tried to reach Mr. Trump but was not able to talk with him because the former president was with his doctors. He said he would try again later in the evening. “Apparently, he’s doing well,” the president said. “I plan on talking with him shortly, I hope.”

Asked if he would characterize the incident as an assassination attempt, he said he had his own opinion but would wait for more facts to come in first.

“The bottom line is that the Trump rally was a rally that he should have been able to conduct peacefully without any problem,” Mr. Biden said. “But the idea, the idea, that there’s political violence or violence in America like this is just unheard of, it’s just not appropriate. Everybody, everybody must condemn it, everybody.”

Mr. Biden was at Rehoboth Beach, Del., where he has a residence, when the shooting occurred. After releasing an initial written statement, he left his residence in his motorcade and headed to a local police department building so he could address the matter on camera.

A White House official told reporters that Biden was briefed on the incident by Kimberly Cheatle, director of the Secret Service; Alejandro N. Mayorkas, the homeland security secretary; and Liz Sherwood-Randall, the president’s homeland security adviser.

A Biden campaign official said the campaign was pausing all outbound communications and working to pull down its television advertising as quickly as possible in deference to the seriousness of the moment.

The president’s statement came after similar expressions by other senior Democrats, including Senator Chuck Schumer and Representative Hakeem Jeffries, both from New York and the party leaders of their respective houses. Prime Minister Benjamin Netanyahu of Israel, Republican congressional leaders and former Presidents George W. Bush and Barack Obama likewise issued comments. Vice President Kamala Harris issued her own statement after the president’s remarks.

Mr. Biden’s relative slowness to issue a statement drew criticism from a Republican senator, Bill Hagerty of Tennessee. “How has the White House said nothing?” he asked on social media shortly before Mr. Biden’s statement was released.

Representative Jared Moskowitz, a Democrat from Florida, called on Mr. Biden to deliver an address from the Oval Office to discuss the situation. “Bring the temperature down,” he wrote on social media. “Remind us we are all Americans.”

There were screams in the crowd as Mr. Trump ducked to the ground before being surrounded by bodyguards. Around me, people were screaming and saying, “Get down, get down, shots fired.” I ducked under the press stand and remained there — until I heard cheering.

I came out and saw Mr. Trump raising his fist defiantly. The crowd, though subdued, cheered. I saw Mr. Trump led down the stairs of the stage to his left, taken immediately to a vehicle. An armored truck also arrived.

I saw police and men in military fatigues yank someone off the bleachers on the other side, in the direction where I’d heard the sound of gunfire.

On those stage-right bleachers, there were people crowded around several people lying prone.

“Trump was just elected today, folks,” I heard a man shout. “He is a martyr.”

It appeared that at least three people were injured.

The Secret Service quickly cleared the press area, moved the crowd out and declared the rally setting a crime scene. State troopers cordoned off a section of the stage-right bleachers with yellow tape.

There was confusion, and the crowd dispersed slowly. On the stage-right bleachers, now empty, I saw blood.

Several medical helicopters arrived after 7 p.m. and departed minutes later, headed south, toward Pittsburgh, about 35 miles away.

Mr. Musk’s formal endorsement may have been surprising in its speed, about 30 minutes after Mr. Trump was sent down to the ground, but not in its content. Mr. Musk, the chief executive of Tesla and SpaceX, has drifted rightward considerably during the Biden era, and he has spent years bashing Mr. Biden and flirting with a formal statement of support for Mr. Trump. Over at least the last year, allies of Mr. Trump have been seeking the endorsement and financial support of Mr. Musk, who has developed a cult following among the online right.

If he so chooses, Mr. Musk could uncork his fortune, measured at $265 billion by Bloomberg, on Mr. Trump’s behalf. Mr. Musk has resisted making major political donations in the past, and has even gone as far as to say that he tries to stay out of politics, despite his conservative rhetoric on immigration and cultural issues.

Mr. Musk has sparred with Mr. Trump in the past. In October 2022, he reinstated Mr. Trump’s account on X, which was then still known as Twitter, after the former president was banned from the platform after the Jan. 6 attack on the Capitol. But he has since used the social platform, where he has nearly 190 million followers, to attack President Biden over policy decisions and his age.

After endorsing Mr. Trump on Saturday, Mr. Musk questioned how the Secret Service did not prevent the attack. In one post on X, he promoted the groundless claim that the agency had allowed it to happen.

“Extreme incompetence or it was deliberate,” he wrote on X. “Either way, the SS leadership must resign.”

He then attacked Kimberly Cheatle, the director of the Secret Service, and Alejandro Mayorkas, the Homeland Security secretary, whose department oversees the Secret Service. Mr. Musk has previously clashed with Mr. Mayorkas over the department’s handling of immigration at the southern border and also said that the secretary “ought to be in jail.”