Police on Thursday arrested a former elementary school principal on suspicion of killing his wife's parents and injuring his two daughters at his home in Fujishima, Yamagata Prefecture, police said.
Shozaburo Amo, 65, is suspected of fatally striking his father-in-law, Ichiro Amo, 87, and his mother-in-law, Matsuko Amo, 82, with a blunt instrument, according to police.
He is also suspected of seriously injuring his daughter Eiko, 30, and slightly injuring his other daughter, Takako, 32, early Thursday, they said.
Police sources said Eiko remains unconscious and is in critical condition.
The police found the elderly couple covered in blood in a bedroom at the family's home.
Investigators said a neighbor heard someone screaming for help and notified authorities around 2:45 a.m.
The couple were taken to a hospital and pronounced dead shortly afterward, according to police.
Amo is reported to have told a paramedic who was treating him for cuts on his wrist, "I did it."
Investigators said they believe Amo tried to kill the whole family before trying to take his own life.
Amo's wife had previously been hospitalized and was not at home when the killings took place.
Neighbors said Amo seemed depressed after suffering a stroke several years ago. -- an ailment that apparently left him with speaking and walking difficulties.