On Tuesday, the FBI released more than 1,500 pages of documents related to its investigation of the 2012 Sandy Hook Elementary School shooting. The gunman, Adam Lanza, pictured above

The FBI on Tuesday released more than 1,500 pages of documents related to its investigation of the 2012 Sandy Hook Elementary School shooting that left 20 first-graders and six educators dead.

The documents include reports by FBI agents who interviewed people about shooter Adam Lanza, who killed himself as police arrived at the school. Large portions of many of the documents were redacted, including the names of the people who spoke to the agents.

The documents offer a window into the early days of the investigation, as agents chased false leads and gathered evidence of Lanza's isolation. A year after the massacre, state police released a final investigative document that concluded Lanza was obsessed with firearms, death and mass shootings, but that the motive may never be known.

One person told an FBI agent that Lanza's mother, Nancy, had become 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 shot his mother to death in their home before going to the school on December 14, 2012. During that time he also dropped to just 85 pounds.

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

A person told the FBI that Lanza essentially became a 'recluse' who shut himself in his bedroom and played video games all day. Above, an evidence photo of Lanza's room after the shooting

One source told an FBI agent that Lanza didn't leave his home (above) for the three months before the shooting, causing his mother to grow concerned

Children flee Sandy Hook Elementary School after the shooting on December 14, 2012

'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.'

Lanza's mother was apparently so dedicated to helping him that they had planned to move cross country to Seattle the following year. She apparently thought that a change of scenery would help her son.

One person said that Lanza owned at least four guns and all were kept in a gunsafe in his closet. Another person said that Lanza had two rifles, but didn't know about him having a pistol.

'Nancy (pictured) would take care of all of Adam's needs,' one source 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 child advocate in 2014 concluded that 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.

Another person told the FBI that 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.

Another person called the FBI to say that her son had been playing Call of Duty: Black Ops online with someone the night before the shooting, who told him to 'watch the news tomorrow'. The documents do not reveal whether the FBI confirmed Lanza was the person who made the comment.

One man said that Lanza had threatened to kill hi mom and attack Sandy Hook four years before the shooting.

'Lanza hated his mother and Sandy Hook, because his mother had worked there,' reads a summary of the interview with the unnamed man. 'Lanza apparently felt his mother loved the students more than she loved him.'

A woman who he met on a website devoted to the Columbine school shooting said Lanza used screen names to honor infamous school shooters like German gunman Tim Kretschmer and Canadian killer Kimveer Gill.

Lanza was 'the weirdest person online,' the woman said.

'He was singularly focused and obsessed with mass murders … Lanza devoted almost all his internet activity to researching and discussing mass murders and spree killings,' read an FBI summary of the December 2012 interview.

'Lanza spoke about school killings and those who committed them with respect and understanding.'

She described him as depressed and cynical with a very negative view on life.

Before the shooting, Lanza had withdrawn from social media, something he told the woman was his 'virtual suicide'.

When she learned he was responsible for the Sandy Hook shooting, the woman said she realized he was 'more f***** up than I thought'

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.

The documents also reveal that Lanza didn't put up a fight when officers responded at the school. When they stormed the school, Lanza turned and ran into a classroom where he committed suicide.

A Newtown resident - who called Lanza 'a troubled kid' - told the FBI that Nancy Lanza said Adam had once hacked into a government computer system and federal authorities — either FBI or CIA agents — showed up at their door.

Nancy Lanza told the person that she had to convince the agents that her son was just very intelligent and was challenging himself to see if he could hack into a government system. She said agents told her that if Adam was that smart, he could get a job with their agency someday.

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