Adam Lanza shot his mother to death on Dec. 14, 2012, and then killed 26 people -- mostly first graders -- at Sandy Hook Elementary before taking his own life.
Investigators found newspaper clippings in his bedroom about school shootings, grisly photos of a dead human and video games that allowed the user to role play a mass shooting. The 20-year-old gunman even kept a spreadsheet ranking mass murders.
On his computer, there were videos showing suicide by gunshot, commercial movies depicting mass shootings, images of him holding a handgun to his head, documents on weapons and magazine capacity and a large amount of materials related to the 1999 Columbine shooting.
Lanza did not allow anyone in his bedroom, not even his mother, the report said. Weeks before the Newtown shooting, his mother, Nancy Lanza, was concerned about her son because he hadn't gone anywhere in three months and would communicate with her by email only, even though they lived in the same house.
According to the report, Laza wrote a book in fifth grade called "Big Book of Granny" that included tales of children being slaughtered and a son shooting his mother in the head.
It was clear that Lanza "had significant mental health issues," but a motive for the tragedy remains unknown.
"The obvious question that remains is: 'Why did the shooter murder 27 people, including 20 children?' Unfortunately, that question may never be answered conclusively," the report said.
Lanza "was under no extreme emotional disturbance for which there was a reasonable explanation or excuse." The hard drive taken from Lanza's bedroom might hold clues, but it was so damaged that data will probably never be extracted from it, according to lead investigator, State's Attorney Stephen Sedensky III.
The report 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.
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."
ABC News and The Associated Press contributed to this report.