Published: Feb 18, 2013 11:26 PM EST
Updated: Feb 18, 2013 11:45 PM EST

(CBS NEWS)- We are learning more about the 20-year-old gunman behind the Sandy Hook Elementary School shooting in Newtown, Connecticut.

Adam Lanza killed 20 first-grade students and six adults two months ago. Now, CBS News has learned authorities believe Lanza was obsessed with another mass murderer. The motive for the murders at the School in Newtown may have been to try to kill more than a Norweigian man murdered a year and a half ago.

In July 2011, Anders Breivik killed 77 people, eight in a bombing near the government center in Oslo, 69 others, mostly teenagers, at a camp on a nearby island.

Two officials who were briefed by Connecticut investigators told CBS News that Lanza was obsessed with Breivik and was trying to top the Oslo shooter's body count. Investigators say Lanza attacked the Sandy Hook school because it was an easy target with a large number of potential victims.

Officials have not revealed details about what led them to connect 20-year-old Lanza to Breivik, but have not managed to retrieve information from Lanza's laptop, which he attempted to destroy.

Investigators found a trove of violent video games in the basement of his home where Lanza practiced his shooting skills. They are now contacting people in the online gaming community, searching for anyone who may have been communicating with Lanza.

Lanza killed himself as Connecticut authorities closed in on the Sandy Hook school, unlike Breivik who surrendered and has been tried and sentenced for the deaths in Norway.

Lanza and his mother made multiple visits to nearby gun ranges where they practiced together.

On December 14, Lanza used one weapon to kill his mother, then headed to the school with three other weapons where he took the lives of 20 children and six adults. All three guns used in the attack were registered to his mother.