A trove of documents recently released by the Federal Bureau of Investigation show that the shooter who killed 58 people at a Las Vegas concert in 2017 was “very upset” about how casinos were treating him.

The documents provide the strongest indication yet of a motive for the deadliest mass shooting in modern American history.

Stephen Paddock, 64, a regular gambler who had a penchant for video poker, killed himself at the Mandalay Bay Resort & Casino before he could be apprehended. The documents provide the most detailed look to date into Paddock’s possible motive and gambling habits, delving into the weeks and years before he fired from his 32nd-floor windows into a crowd of 22,000 people at the Route 91 Harvest Festival.

A fellow gambler told the FBI that “Paddock was very upset at the way casinos were treating him and other high rollers,” noting that roughly three years earlier casinos had started banning high rollers from certain events, hotels and even casinos.

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The gambler described Paddock as a high roller with a bankroll of approximately $2 million to $3 million who preferred playing video poker. The report states that the acquaintance believed the stress about how high rollers were being treated could “easily be what caused Paddock to ‘snap.'”

The Mandalay Bay hotel, Paddock’s acquaintance told the FBI, “was not treating Paddock well because a player of his status should have been in a higher floor in a penthouse suite.”