The friend, who did not want to be identified, told the New York Daily News police appear to believe Adam Lanza had a "psychotic break" before the Sandy Hook Elementary School massacre. Lanza killed his mother Dec. 14 before driving to the school and gunning down 20 students and six staff members, finally taking his own life.
The friend said Nancy Lanza had taken her son to a psychiatrist and Adam would "scream and shout" when his mother urged him to get out of the house.
Nancy Lanza's arsenal included the Bushmaster XM-15 used at the school, two handguns and four rifles.
"They weren't under lock and key," the friend said. "She kept her stuff all together in a closet."
The friend was interviewed by Connecticut State Police.
"They told me they think he had a psychotic break and were asking if Nancy mentioned anything about Adam not taking his medications," the friend told the newspaper.
Notable deaths of 2014 [PHOTOS]