Islamic State claimed responsibility for the mass shooting at a country music festival in Las Vegas, and said the shooter converted to Islam several months ago, according to a group that monitors online terrorist networks.

Yet the FBI later said it determined the shooter has "no connection with an international terrorist group."

The initial claim was based on a report from SITE Intel Group, which said the terror organization claimed responsibility through its Amaq news agency, although ISIS has yet to offer proof of its claim.

"#BREAKING: #ISIS claimed #LasVegas attack, reporting through ‘Amaq that executor is one of its "soldiers" & he converted to Islam months ago," SITE Intel Group tweeted Monday morning.

The shooter, identified as 64-year-old Stephen Paddock, fired down on the crowd of concert-goers at the Route 91 music festival from a room on the 32nd floor of the Mandalay Bay Hotel and Casino.

Police said Paddock committed suicide before law enforcement broke into his hotel room. Clark County Sheriff Joseph Lombardo said law enforcement found at least 10 rifles in Paddock's room, and they believe the gunman acted alone.

"Right now, we believe it's a sole actor, a lone-wolf-type actor," he said.

At least 50 people were killed and more than 400 injured.

SITE noted earlier on Monday that ISIS featured the Las Vegas Strip in a May video, and that an al Qaeda affiliate in 2014 suggested Las Vegas as a target.

ISIS has falsely claimed responsibility for attacks in the past, including a June attack at a casino in the Philippines that left 37 people dead. Police denied the incident was terror-related despite ISIS's claim, and said it was instead perpetrated by a 42-year-old man who had a gambling problem and was in debt.