Police arrested the boy's mother's former dating partner and his wife for assaulting a then-four-year-old boy in Takasaki, Gunma Prefecture, who died two years ago.
The people arrested were Kazutaka Takahara (29), an office worker in Yoge-cho, Isesaki City, and his wife, Chinatsu Takahara (25).
According to police investigation, Kazuki was suspected of being injured in March by beating Shomasa Tsuchiya (4 years old at the time) who lived in the city at his home in Takasaki City, where he lived at that time. Each person is suspected of assaulting Shoya-kun in April.
According to police, the boy died in the hospital he was transferred to in late April.
The boy is a child of Kazuki's former dating partner, and is believed to have been assaulted while he was in custody.
Police have not disclosed the approval or disapproval of the two.
Police arrested the two on suspicion of assault and fatal injury to the boy, but the prosecution did not prosecute them.