The 15-year-old boy arrested on suspicion of murdering three of a family of six in Oita Prefecture rented the violent horror movie "Friday The 13th" before the knife attack, police sources said Friday.
The first-year high school student told his lawyer that he attacked the family because they falsely accused him of being a peeping Tom.
Police are reportedly investigating whether the movie, which depicts the murders of young people, may have influenced him.
According to police, the boy said he decided to murder his neighbors after he happened across his elder brother's survival knife Sunday at his home. He entered the home of the Iwasaki family in the early hours of Monday and stabbed all six of them, killing three, they said, adding that he has expressed remorse for his action.
Police said Friday an emergency call was made by Mai Iwasaki, 16, at 2:52 a.m. Monday on her cellular phone. Police, however, took a long time to confirm that the call had come from the Iwasaki residence.
Consequently, an ambulance was dispatched at 3:31 a.m., with the ambulance actually arriving on the scene at 3:38 a.m., police said.
The attack left Kazumasa Iwasaki's wife, Sumiko, 66, his daughter Tomoko, 41, and Tomoko's son Junya, 13, dead from stab wounds.
Iwasaki, a 65-year-old farmer, remains unconscious and in critical condition with severe wounds to the head.
Mai Iwasaki sustained severe wounds to the back, while Tomoko's other son, Seiya, 11, has wounds to his throat.
The boy under suspicion for the attacks attended the same high school as Mai Iwasaki.
Police said the boy broke into the Iwasakis' two-story house around 2 a.m. Monday through a bathroom window on the first floor.