Safety officials secure the around around Sandy Hook Elementary School following at shooting at the Newtown, Conn. school on Dec. 14. Gary Stoller, USA TODAY
(NBC NEWS) -- Newtown shooter Adam Lanza had no clear motive, according to a long-awaited report on last December's shooting released Monday, but acted alone and planned the rampage that took the lives of 20 children and six school staffers, "including the taking of his own life."
"Many people have asked why the shooter did what he did on December 14, 2012," said the 48-page report, which was published on the state's Division of Criminal Justice website. "Or in the vernacular of the criminal justice system, 'Did he have a motive to do what he did?' This investigation, with the subjstanial information available, does not establish a conclusive motive."
The report also says that there was "no clear indication" why Lanza chose Sandy Hook Elementary School for the shooting, other than that it was close to his home, and that his mental health issues, while they interfered with his ability "to live a normal life," did not affect his criminality.
Read the report here.
The long-awaited summary of the second-deadliest mass shooting in U.S. history, which also left Lanza's mother Nancy dead, omitted much information from the investigative file, including names of child victims, transcripts of 911 calls, some witness statements from children and all crime scene photos.
The release of the report, initially expected over the summer, was pushed back several times amid growing pressure from from authorities -- including Connecticut Gov. Dannel P. Malloy -- to release more information.
Throughout the investigation, State's Attorney Stephen Sedensky III has resisted efforts by the news media and some public officials to release more information related to the shooting.
In March, he ordered police to stop discussing details of the investigation at conferences after the New York Daily News reported that state police Col. Danny Stebbins told audience members at a law enforcement conference in New Orleans that Lanza had created a spreadsheet of mass killings going back 30 years.
He and the town of Newtown also went to court to try and prevent the release of 911 calls from the school or transcripts of them, arguing that making them public could jeopardize the investigation. The state's Freedom of Information Commission ruled in September that the recordings should be provided to the news media, but a prosecutor obtained a stay while he appeals that order. At a hearing in New Britain, Conn. on Monday morning, a judge said he would listen to the recordings and issue a ruling on whether they can be made public.
A Connecticut law passed earlier this year in response to the shooting prohibits the release of photographs, film, video and other visual images showing a homicide victim if they can "reasonably be expected to constitute an unwarranted invasion of personal privacy of the victim or the victim's surviving family members."
Before its release, Sedensky's office and local authorities did everything they could to brace townspeople for the coming onslaught of attention.
Sedensky allowed victims' relatives to review the report at briefings over the previous two weeks.
And the interim superintendent of schools in Newtown advised parents to think about limiting their children's exposure to media reports on Monday and to reach out to mental health professionals if family members need help coping with the contents of the report or the approaching anniversary.
"We all understand that for the children who were directly affected by this tragedy the release of the report and the upcoming anniversary can carry a very personal meaning," read the Nov. 19 letter from Dr. John Reed.
NBCConnecticut, the Associated Press and Reuters contributed to this report.
By Tom Winter and Lisa Riordan Seville, NBC News