Lanza, 20, was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but investigators found no 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 his mother to death inside their home before driving to the school, and committed suicide 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 guns and security.
"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 for his rampage other than that it was close to his home.
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.
In a footnote, Sedensky said a computer drive recovered from Lanza's home might include important evidence but is unreadable, and it is highly unlikely any data will ever be extracted from it.
A timeline released with the report indicates that nearly six minutes passed between the arrival of the first Newtown police officer and the time officers entered the school.
The report said law enforcement officers were operating under the belief there may have been more than one shooter.
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."
The report said the first officer arrived behind the school at 9:39 a.m. Two other Newtown officers then arrived at the school, and gunshots were heard in the background.
The last gunshot officers heard, which is believed to be the suicide shot by Lanza, was heard at three seconds past 9:40. Newtown officers entered the school at 47 seconds past 9:44, according to the report.