A 64-year-old man armed with more than 10 rifles has rained down gunfire on a Las Vegas country music festival, slaughtering at least 59 people and injuring 537 in the largest mass shooting in US history before killing himself.
On Monday, police identified the gunman as Stephen Paddock, who lived in a retirement community in Mesquite, Nevada, and said they had no sense of what prompted his attack.
Police said they had no information about Paddock's motive, that he had no criminal record and was not believed to be connected to any militant group. Paddock 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."
Marilou Danley, 62, who was wanted as a person of interest, is believed to have been out of the country at the time. She reportedly lived with Paddock in a retirement community in Mesquite, about 130 kilometres from Las Vegas.
Federal officials said there was no evidence to link Paddock to international militant groups - despite the Islamic State taking responsibility.
"We have determined to this point no connection with an international terrorist group," FBI special agent in charge Aaron Rouse told reporters.
One US official said there was reason to believe that Paddock had a history of psychological problems.
Lombardo said there were more than 10 rifles in the room where Paddock killed himself. He had checked into the hotel on Thursday.
18 guns were found at his home as well as thousands of rounds of ammunition. Police found several more weapons at Paddock's home in Mesquite, about 145km northeast of Las Vegas, Mesquite police spokesman Quinn Averett told reporters.
"He brutally murdered more than 50 people and wounded hundreds more. It was an act of pure evil," US President Donald Trump said in a White House address.
He ordered flags lowered to half-mast in a national gesture of mourning and said he would visit Las Vegas on Wednesday.
Leaders from around the world expressed shock and sadness at the news.
The shooting broke out on the final night of the three-day Route 91 Harvest festival, a sold-out event featuring top acts such as Eric Church, Sam Hunt and Jason Aldean.
The suspected shooter's brother, Eric Paddock, said the family was stunned.
"We have no idea. 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."
As with previous US mass shootings, the incident sparked anger among advocates for gun control. The Second Amendment of the US Constitution protects the right to bear arms, and gun-rights advocates staunchly defend that provision.