Investigators discovered Sandy Hook shooter Adam Lanza kept meticulous records on previous mass murders on a 7-foot long by 4-foot wide spreadsheet that was so lengthy it needed to be printed on special equipment.

The extensive spreadsheet also contained attempted murders, the New York Daily News reported.

“We were told [Lanza] had around 500 people on this sheet,” a law enforcement veteran told the paper. “Names and the number of people killed and the weapons that were used, even the precise make and model of the weapons. It had to have taken years. It sounded like a doctoral thesis, that was the quality of the research.”

The Daily News’ source was an attendee of the International Association of Police Chiefs and Colonels meeting that took place in New Orleans last week. Among the speakers was Col. Danny Stebbins of the Connecticut State Police, who spoke at length about the Dec. 14 shooting at Sandy Hook Elementary School in Newtown, Conn.

The source, a career police officer, said authorities believe Lanza, 20, thought of the spreadsheet as a “score sheet,” adding that Lanza deeply immersed himself in video games.

“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. 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. That’s what [the Connecticut police] believe,” he said. “They believe that [Lanza] believed that it was the way to pick up the easiest points. It’s why he didn’t want to be killed by law enforcement. In the code of a gamer, even a deranged gamer like this little bastard, if somebody else kills you, they get your points. They believe that’s why he killed himself.”

Connecticut authorities believe the 20-year-old Sandy Hook shooter’s massacre was years in the making.

“He didn’t snap that day, he wasn’t one of those guys who was mad as hell and wasn’t going to take it anymore,” the source said. “He had been planning this thing forever. In the end, it was just a perfect storm: These guns, one of them an AR-15, in the hands of a violent, insane gamer. It was like porn to a rapist. They feed on it until they go out and say, enough of the video screen. Now I’m actually going to be a hunter.”

The spreadsheet was so large that Lanza needed to use a special printer to produce it, the Daily News reported.

Revelations about the spreadsheet emerged a month after it was reported that Lanza looked up to Norwegian mass murderer Anders Behring Breivik, who killed 77 people.

Lanza was “obsessed” with Breivik and chose Sandy Hook to commit the shootings because the school was the “easiest target” with the “largest cluster of people,” officials briefed on the massacre told CBS News. The two mass killers shared a love of video games and guns.