The gunman behind the deadliest mass shooting in modern U.S. history was a 64-year-old Nevada resident who fired out of his room at the Mandalay Bay Resort and Casino before turning his gun on himself, police said.

Las Vegas Police said Stephen Paddock was found dead when a SWAT team breached his room on the 32nd floor of the resort and casino, located across the street from the Route 91 Harvest Festival concert.

He died of a self-inflicted gunshot wound just as police burst their way into the room, Las Vegas Metropolitan Police Department Undersheriff Kevin McMahill told the Las Vegas Review-Journal.

At least 59 people were killed and more than 520 injured in the incident, according to a news release from police.

Paddock checked in as a hotel guest on Thursday, Sept. 28 before the attack, Clark County Sheriff Joseph Lombardo said.

LAS VEGAS SHOOTING: DOZENS DEAD, HUNDREDS SENT TO HOSPITALS IN MASSACRE

Lombardo added that police are investigating how Paddock got the weapons into the hotel and if he had been hiding them.

"We have no idea what his belief system was," Lombardo said. "Right now, we believe he was the sole aggressor and the scene is static."

Responding officers used an explosive device to force the door open into Paddock's room, law enforcement officials told Fox News.

Eric Paddock, the shooter's brother, said that his family was caught off guard by the attack, The Washington Post reported.

"We know nothing. If you told me an asteroid just fell, it would mean the same to me. There's absolutely no sense, no reason he did this," he said. He also told The Post he wasn't aware of any mental illness, alchohol or drug problems in his brother's life.

Eric Paddock said his brother was a multimillionaire who made much of his money investing in real estate, according to the Associated Press. He also said his brother had been an accountant for many years and was unaware of any financial difficulties he may have been having recently.

He also noted that his brother had collected coins as a child.

The last time Stephen contacted the family was when he wanted to check in on them following the passage of Hurricane Irma, Eric said, adding that Stephen had asked how their 90-year-old mother was doing, according to The Washington Post.

In an interview, Eric described his brother as "not a normal guy" who frequently "played high stakes video poker," the Associated Press reported. He added that his brother once "texted me a picture that he won $40,000 on a slot machine."

Eric said he knew his brother owned a few guns but barely used them and was shocked to find out the type of weapons used in the deadly attack.

"He's just a guy who played video poker and took cruises and ate burritos at Taco Bell. There's no political affiliation that we know of. There's no religious affiliation that we know of," Eric said, according to The Washington Post.

Federal law enforcement sources told Fox News that Paddock "was known to local authorities," in Las Vegas. But police in Mesquite, where Paddock lived in a $369,000 home he purchased in 2015, said he never had any run-ins with officers, the Desert Valley Times reported.

Police initially sought a woman believed to be Paddock's girlfriend, Marilou Danley, as a "person of interest." Detectives later made contact with her, and "do not believe she is involved with the shooting on the strip."

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Paddock was carrying some of Danley's identification when he was found dead in the hotel room, Lombardo said.

Paddock and Danley reportedly lived together in Mesquite, Nev., The Washington Post reported. They also reportedly lived in Reno, Nev. for many years.

Some of their neighbors described Paddock as "extremely standoffish" and "aggressively unfriendly," the report said.

One reportedly described him as "weird," and said he "kept to himself."

Other neighbors said they noticed that the couple disappeared for long stretches of time, but Danley had reportedly explained that Paddock was a professional gambler, The Washington Post reported.

A search of Paddock's home there uncovered some weapons and ammunition, Mesquite police told KTNV.

Neighbors in the community were shocked when they heard the news that Paddock was the shooter.

“It’s unbelievable,” Ken Heinly, 73, told the Las Vegas Review-Journal. “This place is just so quiet.”

Property records show Stephen Paddock once owned a home in Viera, Fla., from 2013-2015, the Orlando Sentinel reported.

He had also worked for a predecessor of Lockheed Martin in the late 1980s, and the company is assisting authorities in the investigation, 8 News Now reported.

Fox News' Jake Gibson and the Associated Press contributed to this report.