The group’s Amaq news agency released two statements in Arabic saying ISIS was responsible for the worst shooting in US history.
It claimed the shooter, identified by Vegas police as Stephen Paddock, had converted to Islam “months ago” and called him a “soldier of Islamic Stat”..
The terror group’s claims cannot be verified.
About 50 people died and more than 400 were hurt when the 64-year-old gunman, with an arsenal of at least 10 rifles, fired on a country music festival on Sunday night.
Shooting from the 32nd floor of the Mandalay Bay hotel and casino, he carried out the brutal attack for several minutes before killing himself, police said.
Paddock’s brother told CBS News the shooter had “no religious affiliation, no political affiliation, he just hung out”.
Terror experts have expressed their surprise at ISIS claiming the attack, but pointed out the two statements were released on ISIS’ official primary news source.
Rita Katz, director of SITE Intelligence Group, said it was a “rare event” Amaq released a follow-up statement about Paddock supposedly converting to Islam “months ago”.
The death toll, which police emphasised was preliminary, would make the mass shooting the deadliest in US history, eclipsing last year’s massacre of 49 people at an Orlando night club by a gunman who pledged allegiance to Islamic State militants.
Some 22,000 people were in the crowd when Paddock opened fire, sparking a panic in which some people trampled on others, as law enforcement officers scrambled to locate the gunman.
Shocked concertgoers, some with blood on their clothing, wandered the streets afterwards.
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.
Mr Lombardo, said: ”We have no idea what his belief system was.”
A senior US government official said Paddock’s name was not on any database of suspected terrorists.
Mr Lombardo said there were more than 10 rifles in the room where Paddock killed himself after checking into the hotel on Thursday.
The dead included one off-duty police officer, Mr Lombardo said.
Two on-duty officers were injured, including one who was in stable condition after surgery and one who sustained minor injuries, Lombardo said. Police warned the death toll may rise.
As sunrise approached, police were still finding people who had taken cover during the attack, Lombardo said.
He added: “It’s going to take time for us to get through the evacuation phase.”
Video of the attack showed panicked crowds fleeing as sustained rapid gunfire ripped through the area.
Steve Smith, a 45-year-old visitor from Phoenix, Arizona, who had flown in for the concert, said: “People were just dropping to the ground. It just kept going on.” He said the gunfire went on for an extended period of time.
He added: “Probably 100 shots at a time.
”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 (0500 BST).
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