Thousands of redacted pages from the investigation could shed additional light on the world of the 20-year-old gunman

A planned release on Friday of thousands of pages of police documents from the investigation into last year's school massacre in Newtown could shed additional light on the world of the 20-year-old gunman.

State police said their report totaling several thousand pages would be released at 3pm ET. The report "has been redacted according to law," and includes text, photos and 911 calls received by state police, they said Thursday.

Prosecutors issued a summary of the investigation last month that portrayed the gunman, Adam Lanza, as obsessed with mass murders, but the report concluded that Lanza's motives for the massacre might never be known.

The summary report referred to items found on a computer at Lanza's house that included writings detailing relationships, personal beliefs, a daily schedule, desires, goals and other topics.

Lanza gunned down 20 first-graders and six educators with a semi-automatic rifle at Sandy Hook Elementary School on December 14, 2012, after killing his mother inside their home. He committed suicide with a handgun as police arrived at the school.

The summary said Lanza had "significant mental health issues".

In 2005, Lanza was diagnosed with Asperger's disorder, a mild form of autism that is not associated with violence, and that he lacked empathy for others and behaved strangely, according to the report. Nobody was allowed into his room, he would not touch doorknobs, his food had to be arranged on the plate in a certain way and he changed clothes often during the day.

In fifth grade, Lanza wrote The Big Book of Granny, in which the main character has a gun in her cane and shoots people, and another character talks of liking to hurt people, especially children. The book was among items seized from Lanza's home, but there was no indication he ever handed in the book at school.

Lanza became obsessed with the 1999 bloodbath at Columbine High in Colorado and other mass killings, the report said. He also kept a spreadsheet ranking mass murders.