Investigators have so far been unable to establish the motive of a retired accountant who killed at least 59 people and wounded more than 500 after amassing a weapons cache in a hotel room and opening fire on the Las Vegas Strip.
As America grappled with the deadliest mass shooting in its history, officials reacted sceptically to an Islamic State claim that Stephen Craig Paddock, 64, had carried out Sunday night's massacre on behalf of the jihadist group.
Police said Paddock, who had no criminal record, smashed windows in his 32nd floor hotel room shortly after 10 pm on Sunday and trained bursts of automatic weapons fire on thousands of people attending a country music concert below on the strip.
In video footage of the massacre broadcast on CNN, the rattle of long, sustained gunfire is heard as people scream and scurry for cover. At first they did not know where the shots were coming from.
The Islamic State group claimed that Paddock was one of its "soldiers" but the Federal Bureau of Investigation said it had found no such connection so far and the local sheriff described him as a lone "psychopath."
The terror group said in a pair of statements released by its pseudo-news agency Amaq, citing anonymous sources, that Paddock converted to Islam several months ago and carried out the attack "in response to calls to target states of the coalition" battling IS.
The statement offered no proof of a link with Paddock, nor did it identify him by name.
A US official told CBS News on Monday that intelligence and law enforcement officials were aware of the IS claim, but with the investigation into Paddock's motives continuing and his past being scrutinized by multiple agencies, there were no early signs of any ties to radical Islamic groups or signs of radicalisation.
The FBI says the shooter had no connection to an international terrorist group. The announcement from Special Agent in Charge Aaron Rouse at a news conference Monday comes after the ISIS claimed responsibility for the attack without providing evidence.
Meanwhile, it has emerged that Paddock's father Benjamin Hoskins Paddock was one of the FBI's most-wanted fugitives - a convicted bank robber and con man described in a wanted poster as "psychopathic'' with suicidal tendencies.
The senior Paddock had escaped from prison in 1969, earning him a spot on the FBI's list of most-sought-after and dangerous criminals. According to news accounts, he was not captured until 1978, when he was nabbed while running a bingo parlour in Oregon.
He had been convicted of bank robbery, auto theft and a confidence scam when he escaped from prison in El Paso, Texas. At the time, his son Stephen would have been 15 years old.
Benjamin Paddock, 6-foot-4 and 245 pounds, used a host of aliases, and was also known as "Chromedome" and "Old Baldy," according to his "Wanted" poster.
He remained on the FBI's Most Wanted list until 1977, when he was removed for reasons that aren't clear.
Stephen Paddock's brother, Eric, told reporters outside his Orlando, Florida, home Monday that their father was not around during their childhood.
"I was born on the run,'' he said.
Sheriff Joseph Lombardo said Stephen 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 28 September, they found he had killed himself.
A total of 23 firearms including automatic weapons were found in the hotel room, he said.
A search of Paddock's house in Mesquite, Nevada, 80 miles (130 km) northeast of Las Vegas, recovered 19 additional firearms, some explosives and several thousand rounds of ammo.
Lombardo said investigators had discovered several pounds of an explosive called tannerite in the Mesquite home, as well as ammonium nitrate, a type of fertilizer, in the gunman's car.
He said the latest death toll was 59, while 527 people had been injured.
Lombardo said the authorities had found no manifesto or anything else to explain Paddock's actions. "This individual is a lone wolf and I don't know how it could have been prevented," he said earlier on Monday. "I can't get into the mind of a psychopath at this point."
President Donald Trump denounced what he called "an act of pure evil" and said he would visit Las Vegas on Wednesday.
But the White House pushed back at calls to reopen the US debate on tighter gun controls.
"A motive is yet to be determined and it would be premature for us to discuss policy when we don't fully know all of the facts or what took place last night," Trump's spokeswoman Sarah Sanders said.
The Las Vegas attack is the deadliest shooting in modern US history, exceeding the toll of 49 dead in an attack on the Pulse nightclub in Orlando, Florida in June 2016.
It was also the latest in a series of deadly attacks at concert venues.
In May, 22 people were killed after an Ariana Grande concert in Manchester, England when a suicide bomber detonated a nail bomb in the foyer.
Ninety people were killed in November 2015 at the Bataclan theatre in Paris during a concert by US band the Eagles of Death Metal.
Pope Francis said he was "deeply saddened" by the "senseless tragedy" in Las Vegas, Britain's Queen Elizabeth II offered her "thoughts and prayers" and Russian President Vladimir Putin wrote Trump to voice condolences over the "shocking" crime.