LAS VEGAS (AFP) - America mourned victims of the worst gun massacre in recent US history on Tuesday (Oct 3) as investigators probed the motive behind an apparently senseless attack on Las Vegas concert-goers.

President Donald Trump branded the attacker - who raked a crowd 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, the attack's victims began to be identified in the media, each new story stirring emotions as America once again grappled with calls for reforms to its permissive firearm control laws.

Mr Trump 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."

United States officials have reacted cautiously to a claim by the Islamic State in Iraq and Syria (ISIS) militant 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.

ISIS 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."

Authorities said Paddock, who had no criminal record, smashed windows in his hotel room shortly after 10pm on Sunday and trained bursts of fire on a crowd of 22,000 attending a country music concert below.

In TV footage of the massacre, the sustained rattle of gunfire is heard as people scream and bolt for cover with little idea of where the shots are coming from.

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 were 23 firearms including automatic weapons. Investigators also found another 19 firearms along with explosives and several thousand rounds of ammo at Paddock's house in Mesquite, Nevada, 130km away.

'MIND OF A PSYCHOPATH'

So far, investigators had found no manifesto or anything else to explain the actions of a gunman, but were continuing to hunt and trace every possible clue about a gunman they described as a "lone wolf" and a "psychopath."

"For this individual to take it upon himself to create this chaos and harm is unspeakable," Sheriff Lombardo told a news briefing on Tuesday, saying the shooter's degree of preparation made it clear the attack was extensively premeditated.

Details started to emerge in the media on Tuesday about some of the victims - a kindergarten teacher from California who had married her childhood sweetheart, a Tennessee nurse, a high school secretary from New Mexico.

Stories of heroism also surfaced. Mr Bruce Ure, deputy police chief of the small Texas city of Seguin, was in the VIP section at the concert when the gunfire broke out.

He sheltered from the bullets between two buses, then tended to three people who had been shot. Mr Ure loaded the bleeding strangers into a passing car and rode with them to a hospital.

"They were all crying, and I was too," he told AFP. "They were saying that 'We're going to die, we're going to die,' and I still remember telling them: 'Not tonight, not tonight. Tonight's not your night. You're going to be ok.' Because I truly believed it."

Mr Trump will visit Las Vegas on Wednesday, but the White House has pushed back at calls to reopen the fraught US debate on gun control in the wake of the latest atrocity.

Congress did however shelve a controversial plan to make it easier to purchase gun silencers and make it more difficult to classify certain ammunition as "armour piercing." .

'TWO DOORS FROM A LUNATIC'

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 Mr Eric Paddock said his brother had led an otherwise normal life.

"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.

"We're lost." Paddock had "no religious affiliation, no political affiliation" and was "not an avid gun guy at all," his brother added.

Paddock's neighbours in Mesquite were similarly dumbfounded to discover the killer lived in their midst.

"We could have passed each other going back and forth to the mailbox - and we probably did a number of times - but even now, looking at the news, I don't know him," Mr Rod Sweningson said.

"This is just a quiet, sleepy little community. It just blew me away," he said.

"We've never even thought about locking our doors. We didn't know we lived two doors down from a lunatic."

Paddock had wired US$100,000 (S$136,000) to an account in his girlfriend's native country of the Philippines in the week before the shooting, NBC reported on Tuesday, citing multiple senior law enforcement officials.

But it was not known who was the intended recipient. Ms Marilou Danley, 62, who is Australian by nationality, was overseas at the time of the shooting and investigators said she was not believed to have been involved.

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.