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The suspected gunman behind the Las Vegas massacre made several large gambling transactions in recent weeks, according to multiple senior law enforcement officials and a casino executive.

On several occasions, Stephen Paddock gambled more than $10,000 per day — and in some cases more than than $20,000 and $30,000 a day — at Las Vegas casinos, according to an NBC News source who read the suspect's Multiple Currency Transaction Reports (CTR) and a casino gaming executive.

According to a U.S. statute, a CTR is a Treasury- and IRS-mandated report that casinos have to file when "each transaction in currency involving cash-in and cash-out of more than $10,000 in a gaming day."

It was not immediately clear whether those transactions were losses or wins.

Paddock's brothers were stunned to learn Monday that their sibling was the suspect in the largest mass shooting in modern U.S. history.

Eric Paddock of Orlando, Florida, said he had "no idea" why his 64-year-old brother committed the shooting.

"Mars just fell into the Earth," he said. "We're completely dumbfounded."

Eric Paddock said his brother was retired and was "just a guy" who stayed at hotels, gambled and went to shows.

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Their brother Bruce Paddock, who lives in California, said his family grew up in Sun Valley, California. Their father, Benjamin Paddock, was on the FBI's Top Ten Most Wanted list in the 1970s for robbing banks and was described as psychopathic in an arrest warrant.

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According to the warrant, the suspect's father carried a firearm and was considered "armed and dangerous." It listed his nicknames as "Big Daddy," "Chromedome" and "Old Baldy."

In 1960, The Arizona Republic described Benjamin Paddock as a three-time bank robber who was accused of stealing about $25,000. He was arrested in Las Vegas and "indicted on three counts of robbing Phoenix branches of the Valley National Bank," an archived article said.

The elder Paddock was sentenced to prison in Texas for the robberies.

But six months after his sentencing, he escaped and robbed a bank in San Francisco before being recaptured in Oregon, The Eugene Register-Guard reported in 1978. He had lived in Oregon for several years, evading capture by changing his name and appearance until his arrest, according to the paper.

Bruce, who said he hadn't talked to Stephen in about 10 years, said his brother made money through apartment buildings, which he owned and managed with his mother, who lives in Florida.

"I don't know how he could stoop to this low point, hurting someone else," Bruce said.

He added that his brother, who lived in Mesquite, Nevada, about 80 miles northeast of Las Vegas, was a laid-back, "never-in-a-hurry" type of guy. Bruce said his brother was a law-abiding citizen who never did anything violent prior to the shooting.

Martin J. Kravitz was the lead attorney for Cosmopolitan Hotels when Paddock sued the facility over a slip and fall in 2012.

"He was really kind of bizarre. He dressed slovenly to the deposition," Kravitz said. "He is the kind of personality that's slovenly and careless."

Kravitz said that Paddock did not seem angry and was not the kind of person who would have stood out.

At least 59 people were killed when Paddock opened fire into a crowd of approximately 22,000 at an outdoor country music festival from the 32nd floor of Las Vegas' Mandalay Bay Resort and Casino on Sunday night, police said. Police said at least 527 other people were wounded.

The shooting is not believed to be connected to international terrorism, authorities said.

The suspect was a licensed pilot who owned two planes. He also had at times obtained fishing licenses from Alaska.

It is believed he lived in a retirement community in Mesquite.

Public records show that Stephen Paddock lived a relatively transient life, having 27 different residences in California, Texas and Nevada.

From 1985 to 1988, he worked for a predecessor company of Lockheed Martin as an "internal auditor," public records show.

"We're cooperating with authorities to answer questions they may have about Mr. Paddock and his time with the company," Lockheed Martin said in a statement.

CORRECTION: (Sept 2, 2017, 7:55 p.m. ET) An earlier version of this article misstated the type of license Stephen Paddock obtained in Alaska. He bought a three-day fishing license once in 2009 and again in 2010, not a hunting license, according to officials with the state's Department of Fish and Game.