The report -- a compilation of police and crime scene investigators' reports -- ran almost 7,000 pages and included photographs and videos.
However, it was heavily redacted, with Connecticut State Police Commissioner Reuben Bradford saying it provided "as much detail as possible" on the Sandy Hook Elementary School massacre while protecting victims' privacy.
"Though painful, [the report] will allow those who have been affected by [the shooting] to continue in their personal process of healing, and will provide helpful information that can be put to use to prevent such tragedies in the future," Bradford said in a letter accompanying the release of the report.
Bradford said the investigation has been closed "for administrative purposes," but supplemental reports may be submitted. He said the report issued Friday was redacted to withhold names of children affected by the massacre, as well as the names of most witnesses and "all visual images of the deceased."
The report indicated Lanza shot and killed his mother at their home Dec. 14, 2012 -- one day after she returned from a trip to New Hampshire, where she spoke with a friend about her son's "disabilities" but said she was proud of his accomplishments. After killing his mother, Lanza shot and killed 20 first-grade students and six adults at Sandy Hook Elementary School before shooting and killing himself.
Although federal law prohibits inclusion of Lanza's medical records in the public release, a witness told FBI investigators the day of the shooting Lanza had been diagnosed with Asperger syndrome, a high-functioning form of autism, when he was in sixth grade but "never completely accepted that he had a disease and therefore never took any of his medication he was prescribed," the News-Times of Danbury reported.
"Adam was in complete denial of his disease from diagnosis and there was not on disability because of this denial," the witness said.
The witness said Lanza had been bullied, though "not excessively," because he was socially awkward, and was "finally removed from school" during ninth grade because of stress. Another witness said Lanza became withdrawn during some "episodes" and "was more likely to be victimized than to act in violence against another person."
Investigators were told Lanza became "freaked out" during Superstorm Sandy in October 2012 and would not leave his house.
Connecticut Attorney General Stephen Sedensky III issued a report last month characterizing Lanza as obsessed with mass murders but expressing doubt his motive for the Sandy Hook shooting ever will be known.
The CSP report goes into "much more detail" on Lanza, the News-Times said.
The state police report describes in "a cold clinical way" the death of each Sandy Hook victim and where each body was found.
It includes more of Lanza's medical history and witness recollections of him.
Investigators spent the last few weeks removing personal data from the report, which includes 911 calls.
Victims' families were notified Thursday via email the report would be posted online Friday.