— -- As authorities pick apart the life of Las Vegas shooter Stephen Paddock, they have come across one major thread of suspicious behavior: how he handled his money.
Paddock’s recent financial transactions have become a key focus for investigators looking to learn more about the Nevada man and why he launched the deadliest mass shooting in modern U.S. history.
As ABC News first reported Monday, Paddock recently sent tens of thousands of dollars to someone in the Philippines, where his girlfriend was at the time of the attack, and authorities are still trying to determine who received that money, sources familiar with the matter said.
In the last three years alone, more than 200 reports about Paddock’s activities, particularly large transactions at casinos, have been filed with law enforcement authorities, ABC News was told.
While some of the reports centered around "suspicious activity," most were "currency transaction reports," which casinos are required to file with the Treasury Department's Financial Crimes Enforcement Network when a person withdraws or deposits more than $10,000 in cash.
The number of reports prompted by Paddock's activities reflects -- at the very least -- how routine it was for him to gamble with large sums of money.
In one case, Paddock recently won $40,000 on a slot machine, his brother, Eric Paddock, told The Associated Press.
"That’s the way he played," Eric Paddock said.
One law enforcement official noted "currency transaction reports" are "not necessarily suspicious," especially "if you’re a high roller." As for "suspicious activity reports," 13,736 were filed last year with the Financial Crimes Enforcement Network, according to Treasury Department statistics.
According to the AP, Eric Paddock described his brother as a multimillionaire and said they had business dealings and owned property together.
He said he was not aware that his brother had gambling debts.
A source familiar with the matter told ABC News that Stephen Paddock was a "responsible gambler. He paid his bills, and he came back," adding that there was "no indication of any [financial] stress, any debt, any problems at all."
"He was a very consistent player," the source said. "The notion of his winning or losing $40,000 seems very much within his norm."
The FBI has already briefed some lawmakers about what they’ve found so far as agents dig into Stephen Paddock's past.
"Bottom line, this man was a gambler, but I do not believe there is sufficient evidence to show that he was under stress financially from gambling at the time this incident occurred," the top Democrat on the Senate Homeland Security Committee, Sen. Claire McCaskill, D-Mo., told ABC News.
Paddock’s longtime girlfriend is originally from the Philippines, and she is believed to have been there when he launched his deadly assault in Las Vegas, killing 59 people and injuring more than 500 others attending a country music festival. She is expected to return to the United States Wednesday.
Asked about the reports associated with Paddock, a Financial Crimes Enforcement Network – or "FinCEN" -- spokesperson said the agency "does not comment on specific reports filed by financial institutions," but added, "FinCEN provides access to its database to authorized law enforcement and regulatory users and the data has proven to be extremely useful to investigators."
The FBI declined to comment for this article.