Published: Mar 18, 2013 11:56 PM EDT

Newtown, CT (CBS) -- In the months before the shootings at Sandy Hook Elementary School gunman Adam Lanza compiled extensive research on other mass shootings. He researched the graphic details about the killers, their weapons, and victims.

Sources say Lanza tracked the death tolls of other attacks and viewed them in terms of "scores".   Investigators believe Lanza was trying to achieve a higher score than other mass killers when he attacked the nearby school. Investigators suspect Lanza viewed the school as an easy target filled with people.

As CBS News first reported last month Lanza was particularly obsessed with Anders Breivik, a Norweigian man who killed 77 people in July 2011.  But, Lanza's research went beyond Breivik.  Inside the home Lanza shared with his mother investigators found information about numerous high-profile crimes.

One law enforcement official who was briefed on the case said he's never heard of another killer who did more "planning and organizing".

Investigators already know Lanza practiced his shooting skills.  While there is no record that Lanza purchased any weapons, sources say his mother essentially posed as a "straw buyer".  She bought him guns for his birthdays and took him to nearby ranges.

Lanza also spent countless hours inside his house in a basement gaming room with the windows blacked out.  There investigators found a large collection of violent shooting games. Investigators believe Lanza's fascination with video games and other notorious killers led him to act out his deadly fantasy.