Police arrested an 18-year-old high school student on Monday for allegedly killing his grandparents at their home in Fujikawaguchiko, Yamanashi Prefecture.
The teen, whose name was withheld because he is a minor, admitted to the charges, investigative sources said.
The couple, in their 80s, sustained multiple stab wounds, some significantly deep, suggesting a strong intent to kill. The weapon, thought to be some kind of a knife, was not found.
According to the police, the couple were found dead in their respective bedrooms on the second floor. Only the windows were locked and there was no evidence of robbery.
The couple's younger daughter, the mother of the suspect, found them dead Sunday morning.
Since the murdered 83-year-old man apparently went to a nearby supermarket around 3 p.m. on Saturday, police believe they were killed afterward.