Six bodies, including that of an elementary school girl, were discovered Monday at a home in Miyazaki Prefecture, the police said.
Officers arrived at the home in Takachiho to find a dead woman outside, as well as the bodies of two men, a woman, a boy and the young girl inside the house.
Authorities went to the house after receiving a call from a family member who said all of his calls there went unanswered.
The house was owned by Yasuo Iiboshi, 72, and the body of a man found later Monday under a bridge around 3 kilometers from the house may be that of his son, the police said.
The Miyazaki Prefectural Police said they are treating the investigation as a murder case.