Motive remains a mystery in deadly Las Vegas shooting

Investigators are scouring the home of Stephen Paddock, 64, a Nevada man suspected of opening fire from his Mandalay Bay hotel room into a crowd of concertgoers below on Sunday night.
3:00 | 10/03/17

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Transcript for Motive remains a mystery in deadly Las Vegas shooting
Tom llamas with us tonight. Tom, thank you so much. What would drive someone to take so many innocent lives? That's the question we ask after so many of these mass shootings. Tonight, a portrait of the gunman, now coming into focus. His gambling habit, his family history, and this question -- did he have any ties to foreign groups? Remarkably today, his family said there were no signs that any of this was coming. Here's our chief investigative correspondent Brian Ross. Reporter: As investigators tonight search for clues at his home in a retirement community in Mesquite, Nevada, the motive of 64-year-old Stephen paddock, seen in this photo posted on Facebook by his girlfriend, remains a mystery. We've checked all the federal da databases and local bdatabases and state databases. We had no knowledge of this individual. Reporter: His family remembers him as a man who grew up without conviction. Steve had nothing to do with any political organization, religious organization, no white supremacist, nothing. Reporter: And the FBI says no connection either to any overseas terror group, despite a claim today by ISIS. We have determined to this point no connection with the international terrorist group. Reporter: Yet, for some reason, this otherwise unremarkable man checked into his 32nd floor Las Vegas hotel room last Thursday on a well-planned mission to kill. Assembling a remarkable arsenal. Authorities hope his girlfriend, Marilou Danley, can provide some insight. Authorities says she is in the Philippines, but expected to return to the U.S. Soon. Adding to the mystery, federal officials tell ABC news, paddock sent tens of thousands of dollars in the last few weeks to the Philippines. Paddock's only real passion appears to have been gambling. His brother, Eric, says paddock recently won a $40,000 jackpot, and was a multimillionaire. He was a wealthy guy and he liked to play video poker. He played multi-hundred dollar a hand video poker. Reporter: Six years ago, paddock sued a Las Vegas casino. Video shows him rubbing his leg in pain. An arbitrator found in favor of the casino. Until today, the only known connection to crime was paddock's late father, a violent bank robber who spent seven years in the 1960s as an FBI fugitive, described as psychopathic, suicidal, armed and dangerous. And Brian Ross with us from New York tonight. And Brian, we heard there, in your reporting, the shooter's brother say that he had no political ties, religious ties, no criminal history. Even saying today, no parking tickets. But he did describe his brother as a multimillionaire, who played high stakes poker, but even there, no signs yet tonight of any financial problems? Reporter: Yes, David. Not only did he have real estate holdings, but he's a private pilot. His brother said that he made his fortune in real estate and by gambling and the stock market. He's the kind of high roller the casinos welcome with open arms,

This transcript has been automatically generated and may not be 100% accurate.

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