Five people were found dead in a van in a parking lot on the south side of Mount Fuji in Fujinomiya, Shizuoka Prefecture, police said.
The five people, comprising a man and woman believed to be in their 30s and three boys of elementary school age, were apparently in the same family, the police said after the bodies were discovered Thursday evening.
Murder-suicide is suspected because charcoal pieces and a grill, typically used to cause carbon monoxide poisoning, were found in the van along with a note suggesting it was a case of "family suicide."
The police, who did not make an immediate identification, believe they had been dead several days before they were discovered.
The van was registered in Tokorozawa, Saitama Prefecture, they said.