The 2017 alleged Las Vegas gunman – in what was considered the deadliest shooting in modern American history – was angry at casinos, according to FBI documents recently made public.

Oct. 1, 2017, the FBI contends that Stephen Paddock began opening fire from the 32nd floor of the Mandalay Bay Casino and Resort in Las Vegas, Nevada, onto patrons attending a nearby Route 91 Country Music Festival. In all, 59 people were killed and more than 500 others were injured. 

Authorities said Paddock soon turned the gun on himself. His brain matter was sent to Stanford for testing, and his body was reportedly cremated. 

In hundreds of pages of documents made public online this week, the FBI notes how Paddock was a prolific video poker player who visited the Tropicana Las Vegas once every three months on average. He would usually visit during the week because he thought tourists fed the machines on weekends. During one stay between Sept. 12-14, 2017, he lost $38,000. 

A fellow gambler told the FBI that Paddock "was very upset at the way casinos were treating him and other high rollers," according to the released documents. The interviewee expressed that he was "personally upset and stressed out about the treatment he and other high rollers received in recent years and believed the stress could have easily caused Paddock to ‘snap.’" 

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Debris strewn across concert grounds after Las Vegas mass shooting

Personal belongings seen on the ground at the venue of the Route 91 Harvest Festival in the aftermath of the Oct. 4, 2017, mass shooting in Las Vegas. New FBI docs reveal Stephen Paddock was allegedly angry at casinos. (Robyn Beck/AFP via Getty Images)

The fellow gambler, whose name was redacted from the documents, also told investigators he believed the Mandalay Bay Hotel was not treating Paddock well "because a player of his status should have been on a higher floor in a penthouse suite."

The FBI documents note Paddock had a bankroll of between $2 million and $3 million, and frequented the Atlantis, Peppermill and Tamaric Junction casinos in Reno, Nevada, before being banned from all three, as were other high rollers for winning large quantities of money. 

In previous years, casinos would frequently treat high rollers to free cruises, airline flights, penthouse suites, tours in wine country and nice cars, the FBI documents note, but casinos changed their approach about three years ago and started banning high rollers from certain events, hotels and sometimes certain casinos. 

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Las Vegas mass shooting memorial

A makeshift memorial is seen along the Las Vegas Strip for the mass shooting victims killed at a country music festival. Newly released FBI documents add context about the alleged shooter's gambling history. (Bilgin S. Sasmaz/Anadolu Agency/Getty Images)

The fellow gambler told investigators that Paddock had "never showed any signs of radicalized behavior and never expressed interested in firearms." Paddock "was very intelligent and like all professional gamblers, frequently kept to himself" and played for hours straight, the FBI noted. 

Before gambling became his main source of income, the FBI notes how Paddock previously made money from managing apartment complexes in Los Angeles and Texas and had worked for the U.S. Postal Service and the Internal Revenue Service. He also briefly held a security guard position at an apartment complex in California and a movie production company. 

Another interviewee noted Paddock was "not a political person and never discussed politics." 

The Wall Street Journal first reported about the newly released documents Thursday, noting the paper filed a public records request. 

Mandalay Bay window broken by mass shooter in Las Vegas

Stephen Paddock opened fire from the 32nd floor room at the Mandalay Hotel in Las Vegas, killing and wounding concertgoers below. FBI documents say Paddock might have been angry at casinos. (Mark Ralston/AFP via Getty Images)

In 2019, the FBI’s Behavioral Analysis Unit released a report saying that after a year-long investigation, its panel found "no clear single motivating factor" for why Paddock opened fire on a country music festival. The report said there was also no indication that Paddock was motivated by a grievance against "any specific casino, hotel, or institution in Las Vegas." 

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That 2019 report added that Paddock wanted to kill himself and sought infamy by murdering others.