Las Vegas Gunman Took Elaborate Steps to Hide His Tracks, New Documents Show

As he meticulously planned the Las Vegas attack, the man responsible for the worst mass shooting in modern American history took elaborate steps to stymie the inevitable law enforcement investigation that would follow, according to federal court documents unsealed Friday.

The F.B.I. search warrants shed new light on the degree to which Stephen Paddock, who killed 58 people and wounded more than 500 others after opening fire from a hotel room last October, planned the attack and prepared for the aftermath to come. One of the warrants described how Mr. Paddock “destroyed or tried to hide digital media devices.”

Investigators said he used anonymous communications devices, including a prepaid cellphone to cover his tracks, and employed a “level of sophistication which is commonly found in mass casualty events.”

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

Mr. Paddock’s motive for the attack remains unknown.

The search warrants, which were approved by judges shortly after the shooting, said three cellphones belonging to Mr. Paddock were found in the Mandalay Bay Resort and Casino hotel room, including two that investigators searched and one that they could not unlock without assistance. An F.B.I. agent wrote that he believed “if there were any information related to a potential conspiracy it would be found within” the locked phone, which used a Google operating system.

The search warrants detail how law enforcement focused on Marilou Danley, the girlfriend of Mr. Paddock. She has not been charged with a crime and has spoken to investigators several times. Her lawyer has said she was not aware of Mr. Paddock’s deadly plans. The investigation is continuing and these warrants only show the early stages of the investigation.

“She has been identified thus far as the most likely person who aided or abetted Stephen Paddock based on her informing law enforcement that her fingerprints would likely be found on the ammunition used during the attack,” according to court documents.

She told investigators she occasionally helped him load magazines. The F.B.I. said there was no evidence that she knew of his plans or had been deceptive. But the F.B.I. cautioned at the time that she was still the “subject of intensive review.”

Ms. Danley corroborated much of what had been previously pieced together by investigators but she was adamant that she had no prior knowledge of Mr. Paddock’s intentions to conduct the attack.

According to the documents, it appears that Mr. Paddock used the internet to buy many of the items used in the attack during the previous 12 months, including guns and ammunition.

He spent “significant time and expense prior to the attack purchasing and caching weapons” and other items such as glass cutters and suitcases. Authorities have said he used the glass cutters so he could fire out of his hotel window into the crowd below attending a music concert.

Much of what investigators found in the hotel room is known but the search warrants added additional details. The F.B.I. said there were “preloaded high-capacity magazines” found in suitcases that Mr. Paddock had brought to room. There was also body armor, range finders and a homemade gas mask. Several suitcases were filled with partly loaded high-capacity magazines.

Investigators also revealed that Mr. Paddock may have been treated for “unidentified medical conditions.”

In an affidavit submitted as part of a search warrant application, an investigator said that access to Ms. Danley’s email account could “lead investigators to determine the full scope of Stephen Paddock’s plan and Marilou Danley’s possible involvement.” The authorities also requested information about several Instagram accounts they believed were connected to Ms. Danley.

In their requests for data from Instagram, investigators said they were seeking “evidence showing the possession, use, purchase or sale of firearms, firearms accessories, ammunition or explosives by Paddock.” They also sought information about Ms. Danley’s “state of mind as it relates to the crime under investigation” and the identities of anyone who communicated with her about what would amount to violations of federal weapons laws.

In September, the court records suggested, Mr. Paddock used Amazon to order a holographic weapon sight that investigators believe he ultimately used during the assault in Las Vegas. On Friday night, Amazon listed the same model, which it described as “designed for close-in combat speed and versatility.” The $429 product, Amazon said, “improves target acquisition, boosts accuracy and increases control.”