They said investigators had found “no connection” between the 64-year-old and the Syria-based terror network.
Middle east sources say IS have adopted a practice of claiming random attacks of extreme violence as their own.
IS news agency Amaz said in a statement that Paddock was an IS fanatic.
Their statement said: “He carried it out in response to calls to target states of the coalition.
“The Las Vegas attacker converted to Islam a few months ago. “
But the Bureau’s Special Agent Aaron Rouse said they had “determined to this point no connection to an international terrorist group” despite searching his homes.
Agent Rouse added: “As this investigation continues, we will continue to work with our partners to ensure that this is factually, thoroughly and absolutely investigated.”
Islamic State were the inspiration for last year’s Orlando, Florida, nightclub shooting, which claimed 49 lives, and the 2015 San Bernardino, California, gun attack which killed 14 people.