Adam Lanza studied other mass killers before he stormed Sandy Hook Elementary School in Newtown, Conn., in December, killing 20 students and six adults, officials said.
Officials had previously disclosed finding materials pertaining to Norwegian mass-killer Anders Breivik -- a right-wing ultranationalist who killed 77 people in July 2011 -- in Lanza's home, where he lived with his mother, Nancy Lanza.
Breivik's shooting rampage on Utøya Island, which followed a bombing near government offices in Oslo, targeted teenagers belonging to a left-leaning political party.
Lanza shot his mother in the head four times before proceeding to the school, where police said he used an AR-15-style rifle to do his killing. He shot himself in the head with a pistol.