Investigators are "confident" that no one else entered the Las Vegas hotel suite before a gunman opened fire on a crowd of thousands of concertgoers, killing 58 people in the deadliest mass shooting in modern U.S. history.

Las Vegas Metropolitan Police Department Undersheriff Kevin McMahill said Friday in an interview that police don't believe anyone entered the room at Mandalay Bay prior to the shooting carried out by Stephen Paddock Sunday night.

Earlier, senior law enforcement sources said investigators were puzzled by a charger that did not appear to match any of Paddock’s cellphones. Police have now been able to match all of the cell phone chargers found in the room with multiple cell phones that Paddock had with him.

"He was the only shooter; I'm very confident of that," McMahill said. "I'm also confident that there were no other people in the room leading up to this event."

A motive in the shooting remains elusive. "This agency wants to give answers. We owe it to the victims and the victims' families," McMahill said. "It's really a frustrating piece of this investigation."

At a Friday press conference, McMahill said they did not think there was a second shooter, but he said it was unclear whether or not Paddock had help.

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"We're very confident that there was not another shooter in that room," McMahill said. "What I cannot confirm to you today, and what we continue to investigate is, whether anybody else may have known about this incident before he carried it out."

McMahill said they also found approximately 50 pounds of tannerite and ammonium nitrate — materials that can be used to make explosive devices — in the gunman's car, but it did not appear that Paddock had assembled the material into an improvised explosive device.

Paddock unleashed a barrage of bullets from the 32nd floor of the Mandalay Bay and Casino on Sunday night, killing at least 58 people and injuring about 500 others who were attending a country music festival below. Paddock apparently killed himself, authorities said.

Senior law enforcement officials said Paddock, 64, had researched possible attack locations in Boston and Chicago, including the Lollapalooza festival in Chicago in August.

Law enforcement officials briefed on the investigation also said Friday that they had learned Paddock approached an ammunition dealer wanting to buy a large quantity of tracer rounds at a gun show in Phoenix last month.

Stephen Paddock U.S. Government / via NBC News

The dealer did not have the quantity Paddock wanted, so Paddock did not end up buying anything from him, they said. The dealer told law enforcement that Paddock was neatly dressed and polite, and did not say anything that would raise a red flag. He gave the dealer the impression that he wanted the tracer rounds for recreational use with friends.

Law enforcement learned of the encounter because Paddock had an ammunition box bearing the dealer’s name.

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

McMahill said investigators have not found any connection between Paddock and ISIS, the terror group that claimed responsibility for the attack, a claim it made without offering any proof. The terror group’s claim has been widely discounted.

Investigators have not determined a motive, he said, and have been looking at "everything" including Paddock's personal life, finances, or any political affiliations in searching for a motive behind the attack.

"We have been down each and every single one of these paths, trying to determine 'why,'" McMahill said, adding that the investigation will continue and would focus on facts and not rumors or assumptions.

"In the past, terror attacks or mass murder incidents — motive was made very clear. Very clear in most of those cases by a note that was left, by a social media post, by a telephone call that was made, by investigators mining computer data," McMahill said.

"Today, in our investigation, we don't have any of that uncovered," McMahill said. "I wish we did."