LAS VEGAS — At first, it sounded like fireworks, and then the meaning of the loud, crackling noise began to spread, unevenly, through the huge crowd.
It dawned on people when they heard screams, when they saw bloodied victims collapse around them, or when others stampeded for the exits, trampling some of the people in their way.
Many of the terrified people followed their instincts and crouched or lay flat, not realizing that they remained exposed to a gunman lodged high above them, while others surged into surrounding streets and buildings, leaving behind a litter of drink cups, cellphones, bags and shoes lost in the panic.
By sunrise on Monday, the staggering toll at an outdoor country music festival on a cool desert night was becoming clear: at least 59 people died, the police said, and 527 were injured, either by gunfire or in the flight to safety.
A lone gunman perched on the 32nd floor of the Mandalay Bay Resort and Casino, had smashed the windows of his hotel suite with a hammer, taken aim at a crowd that the police said numbered 22,000 people, and committed one of the deadliest mass shootings in American history.
At least 20 rifles and hundreds of rounds of ammunition were found in the suite, a law enforcement official said. Among the guns were AR-15-style assault rifles, another official said.
The gunman had two rifles, outfitted with scopes, set up on tripods in front of two big windows, said the first official. Both spoke on the condition of anonymity because they were not authorized to divulge details of the investigation.
The authorities did not immediately say what model of weapon or weapons the gunman fired, but on videos posted online by witnesses, the rapid-fire sound indicated that it was fully automatic, like a machine gun.
Among the revelers below at the Route 91 Harvest Festival, Melissa Ayala, 41, said she first realized what was happening when a man near her fell to the ground, blood spurting from his neck.
“It seemed like rapid fire,” she said. “There was blood pouring everywhere.”
She said she and four friends, who had come to the festival from California, were drinking and laughing when they heard the gunfire, which at first they thought was fireworks.
“It was just total chaos,” she said. “People falling down and laying everywhere. We were trying to take cover and we had no idea where to go.”
Her friend, Shami Espinoza, 38, wiped away tears and said it seemed their only choices were “either run and get shot and die, or stay and get shot and die.”
The police identified the gunman as Stephen Paddock, 64, a retiree with no significant criminal history, who liked to gamble and seemed to live a quiet life with his girlfriend in Mesquite, Nev. Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department said SWAT units swarmed into the hotel, starting on the 29th floor, going floor by floor and room by room, before finding the gunman in his room..
“We believe the individual killed himself prior to our entry,” the sheriff said.
Mr. Paddock was in a suite, number 32135, a hotel employee said, giving him sweeping views to the east and north.
Eric Paddock, a brother of Stephen Paddock who lives in Orlando, Fla., told CBS News that his brother was “not an avid gun guy at all,” adding, “if he had have killed my kids, I couldn’t be more dumbfounded.”
“The fact that he had those kind of weapons is just — where the hell did he get automatic weapons?” Eric Paddock asked.
“He has no military background or anything like that,” he added. “He’s a guy who lived in a house in Mesquite and drove down and gambled in Las Vegas.”
He said Stephen Paddock had recently texted him to ask how their mother was faring after Hurricane Irma.
The Islamic State terrorist group, through its news agency, Amaq, claimed responsibility for the attack, saying that Mr. Paddock had converted to Islam months earlier, and had “responded to calls for targeting coalition countries” that are fighting the group in Syria and Iraq. That phrasing usually indicates that the attacker was inspired by the group, rather than directed by it, but such claims have not always proved accurate in the past.
Aaron Rouse, special agent in charge of the FBI office in Las Vegas, said, “We have determined to this point no connection with an international terrorist group.”
The first reports of the shooting came at 10:08 p.m. local time. Officers were overheard on police radio channels reporting that they were pinned down by gunfire. Shortly before midnight, the Las Vegas police reported that “one suspect is down,” and soon after, the police said they did not believe there were any more active gunmen.
Speaking at the White House, President Trump called the shooting “an act of pure evil,” ordered flags flown at half-staff, and said he would travel to Las Vegas on Wednesday.
“The F.B.I. and the Department of Homeland Security are working closely with local authorities to assist in the investigation,” Mr. Trump said, and he praised the performance of the Las Vegas police. “The speed with which they reacted was miraculous and prevented further loss of life.”
Video of the shooting captured nine seconds of rapid-fire, continuous bursts of fire, followed by 37 seconds of silence from the weapon and panicked screaming from the crowd. When the shooting began, the country music artist Jason Aldean continued singing “When She Says Baby” for a few seconds before realizing what was happening and taking cover.
Gunfire then erupted again in at least two more bursts, both shorter than the first.
In the confusion after the shooting, the police also descended on the Ali Baba Restaurant, about a 10-minute drive from the Mandalay Bay, and they also investigated reports of a shooting at the New York-New York Hotel and Casino, not far from the concert ground.
Jake Owen, a country singer who was on stage with Mr. Aldean when the shooting began, told CNN on Monday that it was like “shooting fish in a barrel from where he was.”
“This is not an exaggeration: This shooting was going on for at least 10 minutes,” he added. “It was nonstop.”
Concertgoers described hearing round after round of gunfire. “Everyone was running, you could see people getting shot,” Gail Davis, one of the witnesses, said. “I’ve never been that scared in my life,” she added. “To have this happen, I can’t wrap my mind around it.”
University Medical Center, which has Nevada’s only level 1 trauma center, took in 104 patients, arriving by ambulance and private cars, including four who died, and 12 who were in critical condition on Monday morning. “We had our first rush and it was nonstop,” said Danita Cohen, a hospital administrator.
Ordinarily, the trauma center would taken in eight to 10 traffic accident victims in a night. But the trauma teams regularly train for mass casualty events; the most recent session was led by an emergency responder from the Pulse nightclub shooting in Orlando.
Dignity Health-St. Rose Dominican Hospital received 56 patients at its three campuses, including four who were in critical condition. Most had suffered gunshots, but others had been trampled while fleeing, or had been hurt climbing fences to escape the shooting.
“No one has experienced patient volumes to this level,” said Jennifer Cooper, a hospital spokeswoman.
As survivors poured into streets and buildings surrounding the concert site, and the police and paramedics streamed into the scene, unsure how many gunmen there were, the massacre shut down roads and highways; the police reported closing off about a mile of Las Vegas Boulevard and asked the public to steer clear of the area. Hours later, much of the city remained at shut down.
McCarran International Airport in Las Vegas, just east of the Mandalay, said that some flights destined for the airport were diverted because of police activity, and some of the people fleeing the scene ran to the airport, disrupting operations there. Some survivors reported trying to climb the chain-link, barbed wire-topped security fence around the airport, until firefighters ripped the fence up from the ground, allowing them to crawl under it.
Krystal Legette, who was visiting from New York, and several other people were at Sundance Helicopters office at the airport, waiting for a sightseeing flight around the city, when she said three women burst into the building, screaming, “They’re shooting, they’re shooting.” Then another woman came in, bleeding from a bullet wound in her right arm, and Ms. Legette, a nurse, and three others applied a tourniquet.
More and more people ran into the office, until about 100 people had taken shelter there, she said. A company worker turned out the lights, locked the doors and told everyone to go inside closets and other areas away from the windows.
The hotel itself was placed on virtual lockdown after the shooting, guests said.
“We went into the hotel and they started shutting down casinos,” Todd Price, a guest of Mandalay Bay, told CNN. “We tried to get into our rooms, and they shut down the elevators and started to get everybody out.”
The Route 91 Harvest Festival bills itself as “three days of country music on the Vegas Strip,” and Sunday night’s performance was the last of the festival. The site of the concert, the Las Vegas Village and Festival Grounds, run by MGM Resorts, sprawls over 15 acres and has a capacity of 40,000 people. The festival’s website said this year’s three-day concert was sold out.
In the first hours after the shooting, the police searched for a woman described as “a companion” of the gunman, Marilou Danley. Later, the sheriff said she had been located out of the country, and apparently was not with Mr. Paddock when he checked into the hotel, but that “he was utilizing some of her identification.”
Eric Paddock identified Ms. Danley as his brother’s girlfriend. “We were worried that he might have hurt her, too,” he said.