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.
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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.
The Hartford Courant reported that Lanza fired 152 bullets in five minutes at the school and had hundreds of more rounds with him and in his car. At one point, six children escaped Lanza when his AR-15 jammed or failed to fire, sources said.
The Stamford Advocate, citing an unnamed source, said officials found documents on "virtually every mass murder" in the United States and elsewhere after searching the Lanza home.
The source said they found "a lot of material" in particular on Charles Carl Roberts IV, who attacked an Amish schoolhouse in 2006, killing five girls.
The Associated Press also reported that literature on other mass shootings was found at his house.
Connecticut State Police have kept a tight lid on the investigation, whose search warrant affidavits and applications have remained sealed over concerns that the investigation could be impeded.
Investigators last issued a news release on Feb. 19 to denounce media leaks and to say they would not release more information before the conclusion of the investigation, which did not have a given end date.
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