America mourned the victims of the worst gun massacre in recent US history on Tuesday as investigators probed the motive behind a so far apparently senseless attack on Las Vegas concert-goers.
President Donald Trump branded the attacker -- who raked a crowd of thousands with gunfire from a 32nd-floor hotel room, leaving 59 dead and at least 527 injured -- a "demented man."
But beyond that diagnosis, authorities were at a loss as to why a 64-year-old gambler and retired accountant had hauled a vast arsenal of weapons to the hotel and launched his assault.
Meanwhile, a grim parade of victims began to be identified in the media, each new name stirring emotions as America once again grappled with calls for reforms to its permissive firearm control laws and angst over its pervasive gun culture.
Flags are at half staff to honour those killed and injured in the Las Vegas mass shooting, at the Washington Monument in Washington on Tuesday.
Trump, questioned by reporters as he left the White House to survey hurricane relief in Puerto Rico, was not ready to suggest answers.
"What happened in Las Vegas is in many ways a miracle," he said. "The police department has done such an incredible job, and we'll be talking about gun laws as time goes by."
US officials have reacted cautiously to a claim by the Islamic State group that the shooter, Stephen Craig Paddock, had carried out Sunday night's massacre on its behalf.
Experts cautioned that the group -- under pressure in its Syrian and Iraqi heartlands -- may be trying to rally its supporters with a false claim.
In a statement, IS claimed Paddock was one of its "soldiers" but the FBI said it had found no such connection so far and the local sheriff described him as a lone "psychopath."
Police said Paddock, who had no criminal record, smashed windows in his hotel room shortly after 10:00 pm on Sunday and trained bursts of fire on a crowd attending a country music concert down below.
In footage of the massacre broadcast on TV news, the sustained rattle of gunfire is heard as people scream and bolt for cover with little idea of where the shots were coming from.
"We saw bodies down. We didn't know if they had fallen or had been shot," said Ralph Rodriguez, an IT consultant from Pomona Valley near Los Angeles.
"People started grabbing their loved-ones and just strangers, and trying to help them get out of the way," he said.
Sheriff Joseph Lombardo said Paddock fired through the door of his hotel room and hit a security guard in the leg.
But when a SWAT team stormed the room where Paddock had been staying since September 28, they found he had killed himself.
Inside the room were 23 firearms including automatic weapons, he said.
Investigators also found another 19 firearms along with explosives and several thousand rounds of ammo at Paddock's house in Mesquite, Nevada, 80 miles (130 kilometres) away.
Lombardo said they had discovered several pounds of an explosive called tannerite at the house as well as ammonium nitrate, a type of fertiliser, in his car.
'Mind of a psychopath'
But the gunman's motive remains unclear.
"We're hunting down and tracing every single clue that we can get on his background," the sheriff said at a late-night briefing.
So far, investigators had found no manifesto or anything else to explain Paddock's actions, he said.
"This individual is a lone wolf and I don't know how it could have been prevented," he said earlier.
"I can't get into the mind of a psychopath at this point."
As the investigation continued, details started to emerge in the media about some of the victims -- a kindergarten teacher from California who married her childhood sweetheart; a Tennessee nurse; a high school secretary from New Mexico and a cheerleader, also from California.
On Monday night, there were vigils of solidarity with the dead and the wounded. The Empire State Building went dark, as did the Eiffel Tower and much of the Las Vegas strip itself.
Trump has said he would visit Las Vegas on Wednesday, but the White House has pushed back at calls to reopen the debate on tighter gun controls.
Lombardo said Paddock had apparently used some kind of hammer to smash the window of his hotel room before opening fire on the crowd of some 22,000 people.
'Not an avid gun guy'
According to his brother, Paddock was a high-stakes gambler and their bank-robber father was once on the FBI's 10 Most Wanted list.
But Eric Paddock said his brother had led an otherwise normal life, doting on their mother.
"He liked to play video poker. He went on cruises. He sent his mother cookies," he said.
"We're trying to understand what happened," his brother said.
Paddock had "no religious affiliation, no political affiliation" and was "not an avid gun guy at all," his brother added.
The Las Vegas attack is the deadliest shooting in recent US history, exceeding the toll of 49 dead in an attack on a Florida nightclub in June 2016.
It was also the latest in a series of deadly attacks at concert venues.
Twenty-two people were killed after an Ariana Grande concert in Manchester, England in May when a suicide bomber detonated a nail bomb in the foyer.
And 90 people were killed in November 2015 at the Bataclan theatre in Paris during a concert by US band the Eagles of Death Metal.