The gunman who killed 26 people at a Connecticut school last year had "an obsession" with the 1999 Columbine massacre, an official report has found.
Police said there was evidence Adam Lanza, 20, had planned the shooting, which took fewer than 11 minutes.
But they could not establish why he opened fire at Sandy Hook Elementary School in Newtown on 14 December 2012, before turning the gun on himself.
Lanza acted alone and the case is now closed, state police said.
The report said of Lanza: "He had a familiarity with and access to firearms and ammunition and an obsession with mass murders, in particular the April 1999 shootings at Columbine High School in Colorado."
Twelve students and a teacher were killed when two teenagers opened fire in a planned attack at Columbine.
Gunman's 'dry humour'
Before gunning down 20 children and six adults with a semi-automatic rifle at Sandy Hook, the attacker shot dead his own mother at the family home. She had bought him the weapons used in the rampage.
"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," Monday's 44-page report said.
It noted Lanza's "significant mental health issues", but said "what contribution this made to the shootings, if any, is unknown".
Stephen Sedensky, the prosecutor who drafted the findings, also noted that Lanza had previously "displayed no aggressive or threatening tendencies".
Some people who knew the gunman said he had a dry sense of humour and an appreciation for nature, according to the investigation.
Mr Sedensky noted: "Some recalled that the shooter had been bullied; but others - including many teachers - saw nothing of the sort."
Mass murders spreadsheet
Lanza, who would not allow anyone to enter his bedroom, kept a spreadsheet of mass murders listing information about each shooting, according to the report.
He also had two videos showing suicide by gunshot and photocopied newspaper articles, dating back as far as 1891, relating to the shooting of schoolchildren.
Other findings include:
- Nancy Lanza was concerned that her son would only communicate with her by email, even though they lived in the same house
- While he was in primary school, Adam Lanza wrote a story in which a character kills his mother; it also featured violence towards children
- On the day of the attack, a Texas woman contacted Connecticut police to say her son had interacted the day before while playing a videogame with someone who had said there would be a school shooting
The investigation also outlined the timeline of events that morning.
The attack began shortly after 09:30 when Lanza forced his way into the locked school by shooting through a window near the front doors.
The first call to emergency services came at 09:35 and the last shot, believed to be Lanza firing a handgun at himself, came at 09:40. Most of the deceased were killed in two classrooms.
"In fewer than 11 minutes twenty first-grade pupils and six adults had lost their lives," the report said.
Donna Soto, the mother of Victoria Soto, one of the teachers killed by Lanza, said in a statement that it was still not clear why her daughter and the others died.
"We don't know if we will ever be whole again," she said, "we don't know if we will go a day without pain, we don't know if anything will ever make sense again."
The Newtown shooting prompted a renewed US campaign for stricter firearms controls.
While no legislation was passed at a national level, some states - including Connecticut and Colorado - imposed tougher gun laws; other states loosened such restrictions.