Why Adam Lanza went on his murderous shooting rampage at a Newtown, Connecticut elementary school nearly a year ago may never be known.
That was the conclusion of prosecutors Monday, as revealed in a report that closed out their exhaustive investigation into the Sandy Hook tragedy.
Adam Lanza, 20, killed 20 Sandy Hook Elementary School first-graders, six educators, his mother and himself to death on December 14 of last year.
According to the summary report by the lead investigator, State's Attorney Stephen Sedensky III, there was no indication Lanza would've done this.
Lanza was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but never discussed replicating such massacres.
Sedensky also said there was no clear indication why Lanza chose Sandy Hook Elementary School for his rampage other than that it was close by.
"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 had "significant mental health issues," such as Asperger's disorder, but "what contribution this made to the shootings, if any, is unknown."
Asperger's, an autism-like disorder, is not typically associated with violence.
Lanza "was undoubtedly afflicted with mental health issues; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies."
As for whether this was a response to abuse or bullying? "Some recalled that the shooter had been bullied; but others - including many teachers - saw nothing of the sort."
In a footnote, Sedensky said a computer drive recovered from Lanza's home might include important evidence but is unreadable, having been damaged.
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 initial belief was that there may have been more than one shooter.
The report said the first officer arrived at 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, believed to be the suicide shot by Lanza, was three seconds past 9:40. Newtown officers entered the school at 47 seconds past 9:44.