In the years leading up to the shooting at Connecticut's Sandy Hook Elementary School, gunman Adam Lanza went from a merely shy pre-teen to a mentally ill recluse obsessed with school shootings, according to the report released Monday.
The report by Connecticut State's Attorney Stephen Sedensky said Lanza, 20, was obsessed in particular with the April 1999 shootings at Columbine High School in Colorado.
Lanza shot his mother inside their home before driving to his former elementary school, where he fired off 154 shots with a Bushmaster .223-caliber rifle within five minutes killing 20 children and six adults. He then killed himself with a handgun as police arrived.
Gunman went from a shy pre-teen to a mentally ill recluse obsessed with shooting
According to the report, authorities seized numerous items from Lanza's Newtown home, including a spreadsheet listing mass murders by name and information about the incident.
Also found were computer bookmarks pertaining to firearms, military, politics, mass murder, video games, music, books and ammunition; two videos showing suicide by gunshot; and a five-second dramatised video depicting children being shot.
Sedensky said there is also no clear indication why he chose Sandy Hook as the target for his rampage other than the fact that it was close to his home.
Lanza had significant mental health issues that affected his ability to live a normal life and interact with others but did not affect his mental state for the crimes, he said.
"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 said police went into the school within minutes of the first shots being fired and that along with the actions of teachers inside saved many children's lives.
Since the shooting grabbed national headlines, no major gun control legislation has passed Congress despite a major push by the Obama administration.
Federal efforts to establish background checks for firearm purchases that could limit access for the mentally ill also have made little headway in the face of a powerful gun lobby.