WASHINGTON – The FBI on Tuesday released some 1,500 pages from its investigation into the massacre at Sandy Hook elementary school, painting a dark portrait of the young man who carried out one of the worst school shootings in U.S. history.

Adam Lanza killed his mother in their home in Newtown, Conn., then gunned down 20 children and six adult staff members at the elementary school on Dec. 14, 2012. Lanza, then 20 years old, killed himself that same day.

Virtually all of the FBI’s documents, released in response to Freedom of Information Act requests, are heavily redacted and strip out the names of witnesses and family members. In cases where the substance of the witness interviews are summarized in typed reports, the summaries are often accompanied by the hand-written notes of the interviewing agents.

Here are some highlights:

‘The weirdest person online’

The FBI interviewed a person who regularly communicated with the shooter online. Over the course of nine pages, the interview revealed Lanza’s obsession with mass murder and the 1999 Columbine High School shooting that left 12 students and one teacher dead.

Two years before the Sandy Hook attack, the witness connected with Lanza through postings on a website examining the Columbine incident. Lanza, the witness told investigators, was working on a “spreadsheet” and documenting the details of “hundreds of spree killings and mass murders.”

The witness described Lanza as “the weirdest person online,” who was singularly focused on all aspects of mass murder. “Lanza had a very negative world view,” the witness said.

Describing Lanza as deeply “depressed,” the witness said the shooter seemed to have “no effective coping mechanisms to deal with his depression.”

“He seemed to wallow in it and ride out the low periods by hiding in his room and sleeping for 12 hours or more at a time,” the witness told investigators. “Lanza seemed to have no friends or people he could turn to for support or assistance and did not appear to have any enjoyment of life.”

While they had communicated regularly for more than two years, the witness said the contact abruptly stopped just days before the shooting.

Recounting her reaction to the news that Lanza had been involved in the shooting, the witness told investigators: Lanza was “more f----- up than I thought.”

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Evidence suggests Lanza had an interest in children

On July 29, 2014, investigators released the results of a behavioral analysis of Adam Lanza to law enforcement and victims’ family members.

They found there was evidence to suggest Lanza had an interest in children that “could be categorized as pedophilia” – though there is no evidence he acted on it.

Investigators also found Lanza’s deteriorating relationship with his mother was a significant stressor in the weeks and months before the attack. However, the behavioral analysis found Lanza did not “snap,” but engaged in “careful, methodical planning and preparation” for the attack.

These investigators found no evidence that Lanza viewed his attack as a “video game” or contest, despite speculation in media reports after the shooting that his interest in violent games may have influenced his actions.

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Neighbors say agents previously questioned Lanza for hacking

Years before the Sandy Hook shooting, law enforcement agents visited Lanza's family home after he hacked into a government computer system in ninth grade, his neighbors told FBI agents investigating the Sandy Hook shooting.

Lanza's mother, Nancy, told neighbors at a party that her son made it to the second level of security on a government site. A third layer of security was able to thwart his hack.

Nancy Lanza believed the authorities came from either the FBI or CIA – but neighbors told the FBI that she couldn't remember which agency. When they came to Lanza's house to investigate the potential cybercrime, Nancy Lanza told officials her son was extremely intelligent and he was trying to challenge himself, according to the documents.

Agents then apparently told Nancy Lanza that if her son was that smart, he could have a job with them someday. The FBI Tuesday found no immediate record of a contact with Lanza prior to the shooting.

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'Weirded out' after Hurricane Sandy

In a separate interview conducted just hours after the shooting, an unidentified witness, who appeared to be a family member, discussed Lanza’s long struggle with personal interactions and his diagnosis with a type of autism, known as Asperger Syndrome.

According to FBI documents, Lanza’s mother told the witness that Adam had “weirded out” in the aftermath of Hurricane Sandy, which ravaged a large swath of the northeast coastline just two months before the Sandy Hook attack.

Adam and his mother spent days in the house without electricity, the witness said, because Adam refused to go to a hotel.

– Bill Theobald contributed