A shooter with multiple machine guns fired hundreds of bullets at festival-goers in the deadliest mass shooting in modern US history.
The 64-year-old attacker rained gunfire on a country music festival in Las Vegas from the 32nd floor of the Mandalay Bay hotel.
At least 59 people were killed.
The preliminary death toll, which officials said could rise, eclipsed last year's massacre of 49 people at an Orlando, Florida, nightclub by a gunman who pledged allegiance to Islamic State.
The barrage of bullets fired into a crowd of 22,000 people lasted several minutes, sparking panic. At least 525 people were injured as some fleeing fans trampled each other while police scrambled to locate the shooter.
Police on Monday identified the gunman as Stephen Paddock, who lived in a retirement community in Mesquite, Nevada. They said they believed he acted alone and did not know why he attacked the crowd. The Islamic State militant group claimed responsibility for the massacre, but US officials said there was no evidence of that.
Shocked concertgoers, some with blood on their clothing, wandered streets, where the flashing lights of the city's gaudy casinos blended with those of emergency vehicles.
Police said Paddock had no criminal record. The gunman killed himself before police entered the hotel room he was firing from, Clark County Sheriff Joseph Lombardo told reporters.
"We have no idea what his belief system was," Lombardo said. "I can't get into the mind of a psychopath."
Federal officials said there was no evidence to link Paddock to militant organizations.
"We have determined to this point no connection with an international terrorist group," said Aaron Rouse, the Federal Bureau of Investigation (FBI) special agent in charge in Las Vegas.
US officials discounted the claim of responsibility for the attack made by Islamic State in a statement.
Multiple machine guns
Sheriff Lombardo said there were more than 10 rifles in the room where Paddock killed himself. His arsenal included multiple machine guns, according to a law enforcement official. US law largely bans machine guns.
Nevada has some of the nation's most permissive gun laws. It does not require firearm owners to obtain licenses or register their guns.
Police found several more weapons at Paddock's home in Mesquite, about 145km northeast of Las Vegas, Mesquite police spokesman Quinn Averett told reporters.
The dead in Las Vegas included a nurse, a government employee and an off-duty police officer.
President Donald Trump said he would travel to Las Vegas on Wednesday to meet victims, their family members and first responders.
Mr Trump said the shooting was an "act of pure evil". He later led a moment of silence at the White House in honor of the victims.
The suspected shooter's brother, Eric Paddock, said the family was stunned by the news.
"We're horrified. We're bewildered, and our condolences go out to the victims," Eric Paddock said in a phone interview, his voice trembling. "We have no idea in the world."
He said his brother belonged to no political or religious organizations, and had no history of mental illness. Their father had been a bank robber who for a while was listed on the FBI's "Ten Most Wanted" suspects list.
A nurse, a local government employee, an off-duty police officer and two Canadians were among the dead.
He said the sense of panic was heightened by not knowing who was behind the attack. "We all thought it was a bunch of terrorists or a terrorist cell."
Video of the attack showed panicked crowds fleeing as sustained rapid gunfire ripped through the area.
"People were just dropping to the ground. It just kept going on," said Steve Smith, a 45-year-old visitor from Phoenix, Arizona. He said the gunfire went on for an extended period of time.
"Probably 100 shots at a time," Smith said. "It would sound like it was reloading and then it would go again."
Las Vegas's casinos, nightclubs and shopping draw some 3.5 million visitors from around the world each year and the area was packed with visitors when the shooting broke out shortly after 10 pm local time.
Mike McGarry, a financial adviser from Philadelphia, was at the concert when he heard hundreds of shots ring out.
"It was crazy - I laid on top of the kids. They're 20. I'm 53. I lived a good life," Mr McGarry said. The back of his shirt bore footmarks, after people ran over him in the panicked crowd.
- RNZ / Reuters / BBC