Prosecutors on Monday sent to Oita Family Court a 15-year-old boy suspected of attacking a family of six with a survival knife, killing three of them, officials said.
The Oita District Public Prosecutor's Office gave the court its written opinion that the boy, whose name is being withheld because he is a minor, should be sent to a juvenile reformatory.
The suspect, a first-year high school student, will not stand criminal trial or face criminal charges under Japanese law as he is under 16.
The family court said it will examine the case during the boy's detention period until Sept. 17. The boy is now being held at the Oita juvenile classification office.
The boy's lawyers said he will admit the allegations when he stands before the court. But they said they may ask for a psychiatric test because they see a discrepancy in his stated motives.
According to police, the boy said he attacked the family, who lived in his neighborhood, allegedly because they falsely accused him of peeping into their bathroom.
Later the teen said he killed them because he thought they might tell neighbors he had stolen underwear from their house, police said.
The boy was arrested Aug. 14 shortly after the attack on the family of Kazumasa Iwasaki, a 65-year-old farmer in the town of Notsu, Oita Prefecture.
Iwasaki's 66-year-old wife Sumiko, his daughter Tomoko, 41, and Tomoko's son Junya, 13, died from stab wounds. Tomoko's 16-year-old daughter Mai, her other son Seiya, 11, and Iwasaki himself sustained severe injuries.
The suspect and Mai attend the same senior high school.