A 77-year-old man was arrested Tuesday on suspicion of killing his entire family with a hammer, including a 4-year-old granddaughter, Japanese police said.
Officers discovered the bodies after receiving a phone call from a man who reported killing his family, the official said.
They rushed to the suspect's house in Kashiwa, east of Tokyo, and found the suspect, Yoshio Kiuchi, slumped on the floor and splattered with blood, the official said on condition of anonymity, citing department policy.
They also found the bodies of Kiuchi's 4-year-old granddaughter, 75-year-old wife, 49-year-old son and 44-year-old daughter-in-law, he said.
"A man made an emergency call to police this morning, saying he killed his entire family," the official said.
He said no other details, including the motive, were immediately available.
Public broadcaster NHK said police found a hammer at the house.
Kyodo News agency said the suspect's wife, Tokiko, was found dead in the kitchen wearing an apron, while his son's family was found on the floor in their pajamas.
Under Japanese law, police make arrests and are in charge of an investigation until prosecutors press formal charges, a process that can take weeks.