Adam Lanza, who killed 26 people at Sandy Hook Elementary School in Connecticut 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.
However, they could not establish why he opened fire at Sandy Hook Elementary School in Newtown on December 14, 2012, before turning the gun on himself.
The case is now closed, and no charges will be brought, state police said.
While investigators noted Adam Lanza’s obsession with mass shootings, they said there was not enough evidence to understand a motive.
The report also noted Adam Lanza’s “significant mental health issues” – he was diagnosed in 2005 with Asperger syndrome, an autism spectrum disorder not linked to violence – but said “what contribution this made to the shootings, if any, is unknown”.
Stephen Sedensky, the prosecutor who drafted the report, also noted of Adam Lanza: “Despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies.”
Before gunning down 20 children and six adults with a semi-automatic rifle at Sandy Hook, Adam Lanza shot dead his own mother at the family home.
“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 report said.
It represents just a part of the evidence collected by Connecticut State Police.
A state law passed earlier in the year said some evidence, including certain images or video of the victims, would never be made available to the public.
Police said “a review of electronic evidence or digital media” belonging to Adam Lanza “revealed that the shooter had a preoccupation with mass shootings, in particular the Columbine shootings and a strong interest in firearms”.
Twelve students and a teacher were killed in April 1999 when two teenagers opened fire in a planned attack at Columbine High School in Colorado.
Other documents at Adam Lanza’s home included a spreadsheet with mass murders listing information about each shooting, two videos showing suicide by gunshot and photocopied newspaper articles from 1891 relating to the shooting of schoolchildren.
The report also outlined the timeline of events as state police understands them.
The shooting began shortly after 09:30 when Adam Lanza forced his way into the locked school by shooting through a glass window near the front doors.
The first call to emergency services came at 09:35 and the last shot, believed to be Adam 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 Adam Lanza, said in a statement that it was still not clear why her daughter and the others died.
[youtube a8QO6FgCIFc 650]