NEWTOWN, Conn. -- Police were warned that the Sandy Hook Elementary School shooter had threatened to kill his mother and students before the 2012 massacre, according to newly revealed FBI documents.

The agency on Tuesday released more than 1,500 pages documents on the investigation into the mass shooting in Newtown, Connecticut, that left 26 children and teachers dead.

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

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One woman told investigators that she overheard Adam Lanza threaten his mother. She said that he "had an assault weapon and that she was scared of him." She said she overheard Lanza say that he "planned to kill his mother and children at Sandy Hook in Newtown, Connecticut."

According to investigators' interview notes, she said she called Newtown police and Sandy Hook and told them about the threats, but police said Lanza's mother owned the firearms and there was nothing they could do; they were told to call state police.

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. On the day of the massacre, Adam Lanza fatally shot his mother before going to the school.

The person also told the FBI agent that Adam Lanza never 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."

A report by the Connecticut Office of the 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.

An unidentified woman said she was in contact with Lanza for more than two years on a gaming website dedicated to the 1999 Columbine High School shootings. She told investigators that Lanza kept a "spreadsheet, meticulously documenting the details of hundreds of spree killings and mass murders".

She described him as "depressed" and "isolated" and someone who viewed death as an "escape from his joyless existence."

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.

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

"There are a lot of mentally ill people that are actively seeking to copycat these types of situations," former FBI agent Manny Gomez told CBS News correspondent Anna Werner. "It's going to happen again if people do not step up and try to identify the next Adam Lanza."

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.