Investigators are trying to nail down whether anyone else was in the hotel suite reserved by the Las Vegas gunman during the time he was registered there, multiple senior law enforcement officials briefed on the investigation into the shooting told NBC News.

The investigators are puzzled by two discoveries: First, a charger was found that does not match any of the cell phones that belonged to Stephen Paddock, the man who killed himself inside the Mandalay Bay Resort and Casino after sending a barrage of bullets down on a crowd of 22,000 people below.

And second, garage records show that during a period when Paddock's car left the hotel garage, one of his key cards was used to get into his room.

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There are several possible explanations for these anomalies, the investigators say, but they want to get to the bottom of it.

They are also examining his finances. IRS records show that Paddock was a very successful gambler, earning at least $5 million in 2015. Some of that could be from his other investments, but most of it was from gambling, officials say.

Stephen Paddock U.S. Government / via NBC News

Paddock unleashed a barrage of bullets from the 32nd floor of the Mandalay Bay Sunday night, killing at least 59 people and injuring about 500 others who were attending a country music festival below.

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It was the worst mass shooting in modern American history, and Assistant Clark County Sheriff Todd Fasulo told reporters Monday night that Stephen Craig Paddock was "solely responsible for this heinous act."

His motive for the attack is still not clear. Multiple senior law enforcement officials told NBC News Paddock, 64, had researched possible attack locations in Boston and Chicago, including the most recent Lollapalooza festival in Chicago, which was held Aug. 3 to Aug. 6 in Grant Park.