Lanza, 20, was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but investigators did not find evidence he ever told others of his intentions to carry out such an attack, according to the summary report by the lead investigator, State's Attorney Stephen Sedensky III.
Lanza killed 20 first-graders and six educators with a semi-automatic rifle inside Sandy Hook Elementary School on Dec. 14.
He also shot and killed his mother inside their home before driving to the school, and took his own life with a handgun as police arrived.
The shooting plunged the small New England community into mourning, elevated gun safety to the top of the agenda for President Barack Obama and led states across the country to re-evaluate laws on issues including school safety.
"The obvious question that remains is: 'Why did the shooter murder twenty-seven people, including twenty children?' Unfortunately, that question may never be answered conclusively," the report said.
Sedensky also said there was no clear indication why Lanza chose Sandy Hook Elementary as the target for his rampage other than the fact that it was close to his home.
In a footnote, Sedensky said a computer drive recovered from Lanza's home might include potentially important evidence but is unreadable, and it is highly unlikely any data will ever be extracted from it.
The report said Lanza had "significant mental health issues" - in 2005, he was diagnosed with Asperger's disorder - but "what contribution this made to the shootings, if any, is unknown."
Asperger's is an autism-like disorder that is not associated with violence.
Lanza "was undoubtedly afflicted with mental health problems; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies," Sedensky wrote.
"Some recalled that the shooter had been bullied; but others - including many teachers - saw nothing of the sort."
Donna Soto, the mother of slain teacher Victoria Soto, said in a statement that nothing could make sense of the shooting.
"Yes, we have read the report, no, we cannot make sense of why it happened. We don't know if anyone ever will," Soto wrote. "We don't know if we will ever be whole again, we don't know if we will go a day without pain, we don't know if anything will ever make sense again."
Sedensky has gone to court to fight release of the 911 tapes from the school and resisted calls from Connecticut's governor to divulge more information sooner.
The withholding of 911 recordings, which are routinely released in other cases, has been the subject of a legal battle between The Associated Press and Sedensky before the state's Freedom of Information Commission, which ruled in favor of the AP, and now Connecticut's court system.
Superior Court Judge Eliot Prescott said Monday he will listen to the 911 recordings from the school before ruling on whether they can be publicly released.
If the recordings are released, the AP would review the content and determine what, if any, of it would meet the news cooperative's standards for publication.