A 20-year-old member of a yakuza syndicate, who was arrested in Tokyo on Wednesday over the death of his girlfriend's 3-year-old son, is believed to have previously abused the boy, police said Thursday.
The Metropolitan Police Department sent the case to prosecutors the same day.
Naoya Nagatomi, a member of a group affiliated with major yakuza syndicate Sumiyoshi-kai, was arrested Wednesday for repeatedly hitting and kicking Ayato Arai on Monday.