NEWTOWN, Conn. -- The FBI has released more than 1,500 pages documents on the investigation into the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, that left 26 children and teachers dead.

Many of the documents released Tuesday had large sections redacted. They include reports by agents who interviewed people about shooter Adam Lanza, who killed himself as police arrived at the school.

An unidentified person told an agent that Adam Lanza's mother, Nancy Lanza, was concerned about him a month before the shooting because he had become a "shut-in" who hadn't gone anywhere in three months. Adam Lanza fatally shot his mother before going to the school.

The person also told the FBI agent that Adam Lanza never completely accepted that he had Asperger's syndrome, a condition on the autism spectrum, and never took any medication he was prescribed.

"Nancy would take care of all of Adam's needs," the person said. "However, she never cleaned his room, nor was allowed in his room. Adam's room was his personal space that no one else was allowed into."

Several residents told authorities they received death threats after the shooting in phone calls from someone identifying himself as Adam Lanza.

Another person interviewed said Nancy Lanza told them her son hacked "into a government computer system" and federal agents showed up to their home. They said Nancy Lanza believed the agents were from the CIA and FBI and they said if her son was smart enough to hack the computer he could have a job with them someday.

A report by the Connecticut child advocate in 2014 concluded that Adam Lanza's autism spectrum disorder and other psychiatric problems did not cause or lead directly to the massacre. The report said Nancy Lanza rejected recommendations from Yale psychologists that her son be medicated and undergo rigorous treatment as a child for anxiety and other conditions. It also said Adam Lanza, his parents and educators contributed to his social isolation by not confronting his problems.

In the FBI documents, one person told investigators that Adam Lanza essentially had become a "recluse" who shut himself in his bedroom and played video games all day. The person said Lanza had no friends, was computer savvy and became very interested in firearms.

Lanza shot the children and educators with an AR-15-style rifle that was legally purchased by his mother, who took her son to shooting ranges, authorities have said.

© 2017 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.