HARTFORD, Conn. — There was evidence the Newtown school shooter had an interest in children that could be categorized as pedophilia, but there was no proof he ever 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. Adam Lanza fatally shot 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 behavioral analysis unit wrote. “The shooter was fascinated with past shootings and researched them thoroughly. The shooter shared many similar characteristics and behaviors with other active shooters.”

The behavioral analysis unit document did not say exactly what evidence there was that Lanza had a pedophilic 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 he 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 also believed that mass murders were a symptom of a broken society and he may have believed he was “saving” children from the “harmful influences” of adults during the school shooting.

The documents also 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.

That report also mentioned pedophilia. In it, state investigators said they had found a file on Lanza’s computer that they described as “advocating pedophiles’ 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.