New details are shedding light on the intent of the man who committed the worst mass shooting in modern American history.

Stephan Paddock, the gunman who opened fire on more than 20,000 country fans last October at the Route 91 Harvest festival in Las Vegas, conspired for 12 months prior to the shooting, gathering weapons, ammunition and accessories and scheming to cover up his eventual deadly attack.

According to the New York Times, federal court documents unsealed on Friday (Jan. 12) indicate Paddock employed a “level of sophistication which is commonly found in mass casualty events.”

“Paddock planned the attack meticulously,” the F.B. I. said, “and took many methodical steps to avoid detection of his plot and to thwart the eventual law enforcement investigation that would follow.”

Not only did he attempt to hide or destroy digital media, but also he used at least three anonymous communications devices to cover his tracks. Additionally, the documents state he bought many of the items used in the attack online, including guns and ammunition. Glass cutters, suit cases, a gas mask and body armor were also found in his Mandalay Bay hotel room.

Paddock killed 58 people and wounded more than 500 others before he turned the gun on himself.

His motive for the attack remains unknown.

To read more from the recently unsealed federal court documents, visit the New York Times.

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