A 17-year-old boy was arrested in Kimitsu, Chiba Prefecture, on Sunday on suspicion of killing his grandmother in what appears to be a double murder.
The high school student turned himself in Saturday evening, saying he killed his grandparents. Investigators later found the bodies of his grandfather, 67, and grandmother, 64, in their house.
He was initially arrested for allegedly murdering the grandmother, apparently on Wednesday, but additional charges are expected to be filed.
The teen, who lives with his mother in Kimitsu, often visited his grandparents alone. He told investigators he was stressed out by school problems and could have killed anyone.
"I didn't hold a grudge against my grandparents nor did I have trouble with them," he was quoted as saying.
The bodies of the couple, dressed in pajamas, were found separately on the first and second floors with stab wounds. Their heads appeared to have been beaten repeatedly with a blunt object, police said.
Investigators also found a blood-stained pickax, knife and hammer at the house.
When the teen attended school on Tuesday, the last day of the semester before winter break, nothing looked out of the ordinary, high school sources said.