There was evidence the Newtown school shooter had an interest in children that could be categorised as paedophilia, but there was no proof he acted on it, according to FBI documents released Tuesday.

The records were among more than 1,500 pages of documents released by the FBI in connection with its investigation of the December 2012 shooting at Sandy Hook Elementary School, where Adam Lanza killed 20 first-graders and six educators before killing himself as police arrived.

The records also say there was evidence Lanza began contemplating the attack as early as March 2011.

“The shooter did not 'snap,' but instead engaged in careful, methodical planning and preparation,” the FBI's behavioural analysis unit wrote. “The shooter was fascinated with past shootings and researched them thoroughly. The shooter shared many similar characteristics and behaviours with other active shooters.”

The behavioural analysis unit document did not say what evidence there was that Lanza had a paedophilic interest in children. But another document says an unidentified woman told the FBI that Lanza said adult-child sexual relationships could be “possibly beneficial to both parties.”

The woman, who said she had an “online relationship” with Lanza for more than two years before the school shooting, said Lanza did acknowledge that adult-child sexual relationships could be “unhealthy” and did not express any personal sexual interest in children. She said Lanza believed he might be asexual.

She also told the FBI that Lanza compiled a spreadsheet that meticulously documented hundreds of mass murders and spree killings, but she didn't believe he would carry out a mass killing. She said Lanza believed mass murders were a symptom of a broken society and may have believed he was “saving” children from the “harmful influences” of adults during the school shooting.

The documents include reports by FBI agents who interviewed people about Lanza. Portions of many of the documents were redacted, including the people's names.

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 his motive may never be known.

That report also mentioned paedophilia. In it, state investigators said they found on Lanza's computer a file they described as “advocating paedophiles' rights and the liberation of children.” They also said they found a screenplay describing a relationship between a 10-year-old boy and a 30-year-old man.

One person told an FBI agent that Lanza's mother, Nancy Lanza, 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 14 December 2012.

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

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

Another person told the FBI that Lanza essentially had become a “recluse” who 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 legally purchased by his mother, who took him to shooting ranges, authorities have said.

Sandy Hook elementary school shooting 2012 Show all 5 1 /5 Sandy Hook elementary school shooting 2012 Sandy Hook elementary school shooting 2012 con0.ap.jpg Scenes of shock and devastation at the Sandy Hook Elementary School in Newtown, Connecticut AP Sandy Hook elementary school shooting 2012 v2Connecticut.jpg Picture from local newspaper the Newtown Bee shows Connecticut State Police lead children from the Sandy Hook Elementary School AP Sandy Hook elementary school shooting 2012 con1.reut.jpg The scene outside the primary school following the incident REUTERS Sandy Hook elementary school shooting 2012 con2.reut.jpg Concerned parents picking up their children following the shooting REUTERS Sandy Hook elementary school shooting 2012 obama.reut.jpg Obama wipes away a tear during the press conference REUTERS

A Newtown resident told the FBI that Nancy Lanza said Adam Lanza once hacked into a government computer system and federal authorities showed up at their door.

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