IRS records show that the Las Vegas gunman who killed dozens in a hail of gunfire this week was a successful gambler who earned millions of dollars in 2015, according to a report by NBC News.

Stephen Paddock, who killed at least 58 people and injured more than 500 when he opened fire on a crowd of festival attendees on the Las Vegas strip, earned at least $5 million in 2015, the report said. The report noted that most of that income came from gambling, although some could have come from other investments.

The investigation by law enforcement initially raised questions about whether Paddock was staying alone in his hotel room, NBC reported. However, law enforcement later said that they were "confident" that no one else entered Paddock's hotel room prior to the shooting.

"He was the only shooter; I'm very confident of that," Las Vegas Metropolitan Police Department Undersheriff Kevin McMahill said, according to NBC.

The New York Times reported earlier this week that Paddock was not well known among Las Vegas's high stakes gambling crowd, though he would occasionally make tens of thousands of dollars in a single sitting. Because of his prolific gambling, Paddock was staying at the Mandalay Bay Hotel free of charge last weekend, The Times reported, citing a source familiar with Paddock's history.

Law enforcement in Las Vegas were not available to comment Friday afternoon. Officials also did not immediately respond to an emailed request for comment.