The unidentified law enforcement officer told the New York Daily News Lanza's spreadsheet of names, weapons and body counts from previous mass murders and attempted killings, compiled over a number of years, "sounded like a doctoral thesis. That was the quality of the research."
On Dec. 14, Lanza, 20, shot his mother, 20 children and six adult staff members of a Newtown elementary school, killing himself afterward.
The source said Connecticut law enforcement officials believe the spreadsheet was "a score sheet," the Daily News reported Sunday.
"This was the work of a video gamer, and that it was his intent to put his own name at the very top of that list," the source said. "They believe that he picked an elementary school because he felt it was a point of least resistance, where he could rack up the greatest number of kills."