A 24-year-old woman and her 20-year-old boyfriend have been arrested on suspicion of fatally assaulting the woman's 22-month-old son in their apartment in Izumisano, Osaka Prefecture, according to local police.
Officials at Izumisano Police Station suspect that Kotomi Manda and her boyfriend, Masahiko Shibata, beat her son, Takahiro, on Friday and Saturday.
The boy, whose face, legs and back were extensively bruised, died at a hospital late Tuesday night, according to police.
He had been in a coma after suffering brain and other damage.
The couple were arrested Sunday evening after a rescue worker who took the boy to the hospital alerted police to the boy's condition.
Initially, Manda told the hospital that her son had "had a fight" with his sisters.
Shibata later told police that he slapped the boy "to teach him a lesson" after he allegedly failed to sit up straight, and Manda said she and her boyfriend beat him to "chase evil spirits away," police said.
Manda, a divorced mother of three, is also suspected of abusing her 3-year-old daughter, although she has told investigators that the girl had been abused by Shibata, according to police officials.
The 3-year-old and her 5-year-old sister were taken to a local child protection center Monday, but the younger daughter was later hospitalized after officials found dozens of bruises on her body as well.
The younger daughter had suffered a subcutaneous skull hemorrhage.
Investigators said Wednesday the girl also had 126 burns.