A woman in Funabashi, Chiba Prefecture, was arrested along with her partner Friday for allegedly physically abusing the woman's 4-year-old son, resulting in his death.
Local police identified the woman as Chika Kojima, a 20-year-old driver, and the man as Toshikazu Kanno, a 27-year-old company employee.
Kojima allegedly struck her son, Kakeru Tano, in the face so hard that his head smashed into a bookshelf at around 7 p.m. Tuesday. Police said the boy appeared to have suffered severe head trauma.
Kojima reportedly told police that she beat her son because he would not stop crying, admitting to abusing him.
Police are investigating to determine whether the death was caused by abuse and are questioning both Kojima and Kanno.
Kanno also is suspected of striking the boy several times in mid-July, bruising the left side of his skull.
The Funabashi fire department said it responded to an emergency call from Kojima around 11:20 a.m. Thursday, in which she claimed her son had been found submerged in the bathtub.
When an ambulance arrived, Kojima was standing in front of her home, holding her child, the department said, adding that the boy did not appear to be breathing.
A doctor called police after finding bruises on the child's head and stomach, the department said.
Kojima had her son with her former husband. The boy had been living in Kanno's home since spring 2001, police said.