Police Precinct Part 13

On a quiet spring day in the suburbs, a stray dog found the corpse of a baby. Three days after his death, the blood group of the victim was determined. Detectives Hayashi, Kaneko, Nagata and Yamagata begin their investigation, suspecting the parents of the murdered child. They use a towel, baby diapers and bruises on the baby's body as clues in the investigation. Who killed the baby?