The Federal Bureau of Investigation (FBI) has declassified a 1500-page document about its investigation into the shooting spree that occurred at Sandy Hook Elementary School in Newtown, Connecticut.

In December 2012, 20-year-old Adam Lanza shot to death his own mother before going on a rampage that killed 20 elementary students aged 6 to 7, six adult staff members and finally, himself.

The document, which was revealed in response to Freedom of Information Act requests, is still heavily redacted, but it gives an insight into Lanza's motive in killing innocent children.

BBC reported that the gunman, an introvert and computer geek, held National Rifle Association safety qualifications. He and his mother visited shooting ranges in the past and all of five weapons and ammunition he used were legally purchased.

He was pulled off school in his mid-teens after being excessively bullied. He was diagnosed with Asperger's syndrome, one that is related to autism, but he never accepted the diagnosis and refused treatment.

According to ABC News, a month before the shooting, a family friend said his mother had been concerned with how Lanza had kept to his room and computer for three months.

His most dominant online activity was researching previous mass shootings, including the one that happened in Columbine High School in 1999. It was through a role-playing website where visitors adopt the roles of Columbine shooters Eric Harris and Dylan Klebold that Lanza met a woman, whose name was redacted in the files, with whom he shared an "online relationship" for more than two years.

The woman described Lanza as the "weirdest person online," telling authorities that he was very cynical about the world, but that she never thought he was capable of killing himself or even other people.

However, the FBI found that Lanza had an obsession with mass shootings, and even compiled a very detailed spreadsheet of previous mass murders. The document concluded that he had been planning the act since March 2011.

"The shooter did not 'snap,' but instead engaged in careful, methodical planning and preparation," the report said.

The documents also support a 2014 report that said, "The shooter had an interest in children that could be categorized as pedophilia," although they never found any evidence to indicate that he acted on any sexual feelings towards children.

According to Lanza's online friend, he believed adults were "improperly controlling" children, leading him to feel "pity" for them.

She suggested that in Lanza's mind, killing those kids was his way of "saving" them from harmful influences, which he believed are their parents and teachers.