(CBS News) Monday night, CBS News broke the story that Adam Lanza, the killer at Sandy Hook Elementary School, had an obsession with a mass murder that occurred the year before.
The Hartford Courant newspaper added to that, reporting that investigators found clippings about the earlier killing in Lanza's home.
In December, Lanza killed his mother, 20 first graders and six members of the school staff before killing himself. More details have emerged on the motive.
Sources say material about Norweigen mass murderer Anders Breivik was found in one of Lanza's rooms. Breivik killed 77 people in 2011 in a bombing and shooting attack in Norway.Now police are pursuing a theory that Lanza viewed himself as being in direct competition with Breivik.
Sources say Lanza attacked nearby Sandy Hook Elementary School because it was an easy target with a large number of people. Investigators believe a rapid police response forced Lanza to take his own life and end the massacre earlier than intended. Hundreds of rounds of unspent ammunition were found with Lanza.
Connecticut State Police strongly deny they've established any hard connection between Lanza and Breivik and say any reports about a possible motive are premature.
"The unfortunate origin of unsubstantiated details of this case are both hurtful and, many times, inaccurate," Lt. Paul Vance said in a statement.
But two officials who have been briefed on the investigation say Connecticut officials have privately indicated they believe Lanza was obsessed with Breivik.
CBS News correspondent John Miller reports how Lanza descended into his own world by blacking out windows.
It's also clear Lanza was heavily involved with violent video games. A large collection of games was discovered in a search of his home. Investigators believe in the months before the attack Lanza spent countless hours alone in a basement gaming room with the windows blacked out.
CBS News also knows Lanza had blacked out the windows of his bedroom. He was a young man who sought complete isolation, and who died without explaining his actions.