The paperwork "has been redacted according to law," and it includes text, photos, videos and recordings of 911 calls received by state police.
In a letter accompanying the report, Reuben F. Bradford, commissioner of the state Department of Emergency Services and Public Protection, wrote that much of the identifying information involving children and many witnesses was being withheld.
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 Dec. 14, 2012, after killing his mother inside their home. He committed suicide with a handgun as police arrived at the school.
To try to figure out the motive, investigators said, they interviewed members of Lanza's family, teachers and others. They said they also tried within the limits of privacy laws to gather information on his medical treatment.
Lanza "was undoubtedly afflicted with mental health problems; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies," it said.
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.
The report also said that in 2005, Lanza was diagnosed with Asperger's disorder - an autism-like condition that is not associated with violence - and that he lacked empathy for others and behaved strangely. Nobody was allowed into his room, not even to clean, according to the report. It said Lanza also disliked birthdays, Christmas and holidays and did not like to have his hair cut.
He also wouldn't touch doorknobs, his food had to be arranged on the plate in a certain way, and he changed clothes often during the day. He was a loner at school and was repelled by crowds and loud noises.