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 on Monday discovered the bodies of two more people, who reports said appeared to have been battered to death. All five of the dead 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 is a traditional Japanese form, a three-line verse of 17 syllables in a five-seven-five arrangement. It customarily evokes natural phenomena, frequently as a metaphor for human emotions.
The haiku reads: “Setting a 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.
The mass-selling Yomiuri Shimbun said the suspect had once boasted to neighbours that he would be immune from prosecution if he killed people because he is on medication. It was not clear what the man’s condition might be.