Sandy Hook Report: Grim Details, No Motive

Extensive police report includes images of shooter Adam Lanza's room, video games.
3:00 | 11/26/13

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Transcript for Sandy Hook Report: Grim Details, No Motive
Now, to the new clues just revealed about the mind of a madman. Details emerging about the newtown school massacre gunman. A report about the rampage is out. And abc's brian ross is here with the latest on that. Reporter: Good morning, robin. The investigation was closed. But authorities saying they were unable to determine the motive of the shooting that led to so many being killed. Sandy hook school. Caller is indicating, she thinks there's something shooting in the building. Reporter: The report found that the shooting rampage took only 11 minutes. The shooter's apparently still shooting. Reporter: As to why 20-year-old adam lanza targeted the school and killed himself, the state prosecutor said, his investigation does not establish a conclusive motive. What's clear is that the culprit is adam lanza. And all those who wish to blame drugs or video games or lax gun laws or even nancy lanza, are misplaced. Reporter: For the first time, officials released photos taken inside lanza's home, where he first killed his mother, nancy. His bedroom and computer room, with the windows covered by black garbage bags were the lair of a recluse. There was a shooting range target with holes in it. Video game, that authorities say he spent hours playing. An open gun safe, with a teddy bear on top. And news clippings, dating back TO THE 1890s, THAT SHOWED AN Obsession with mass murders, in particular, the columbine shootings. His entire world was I think planning some event like this shooting. Reporter: Officials say lanza communicated online with others interested in mass murders. But he destroyed his hard drive before leaving on his deadly mission. Out of consideration for the families, other material from the investigation that might ordinarily be made public, is still being withheld, including 911 calls and photos of the young victims. To the relief of nicole, whose son, dylan, was among those killed. She talks with lara spencer. It's not in anybody's interest. It's not how I want dylan to be remembered. Reporter: The report says lanza had significant mental health issues. And in tend, only communicated with his mother by e-mail. But the prosecutor concluded lanza was well enough to plan an attack, including the taking of his own life, all of the guns legally purchased by his mother. In many ways, it's hard to believe we're coming up on a year since that happened. Brian, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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