The five victims represent a third of the population of the hamlet in western Yamaguchi prefecture, where police on Sunday found three corpses after two houses were burned to the ground.
Investigators yesterday discovered the bodies of two more people, who reports said appeared to have been battered to death. All five victims were in their 70s or 80s.
The chief suspect is a 63-year-old villager at whose home police found a "haiku" poem stuck to the window.
The haiku reads: "Setting on fire -- smoke gives delight -- to a country fellow".
The man, whose name has not been made public, has a reputation in the village as a trouble-maker, reports said.