Photo/Illutration The Metropolitan Police Department headquarters in Tokyo’s Chiyoda Ward (Asahi Shimbun file photo)

Tokyo police on Dec. 3 arrested the boyfriend of the mother of a 3-year-old boy whose death after an apparent attack in September was disguised as an accidental drowning.

Yuji Watanabe, a 34-year-old company employee in Akishima in western Tokyo, is suspected of inflicting injury resulting in death.

He has denied the allegation, saying, “I did not do anything,” according to police.

Police suspect Watanabe unleashed his attack against Ryutaro Yamada in his abdomen at their residence in Tokyo’s bayside Toyosu area around Sept. 28.

At that time, Watanabe was living with Ryutaro, the boy’s mother, whom the suspect was in a relationship with, and his own 1-year-old son.

Watanabe called for an ambulance around 1:25 p.m. on Sept. 28. Ryutaro was taken to a hospital and pronounced dead around 6:40 p.m. the following day.

Police said Watanabe explained that he and Ryutaro were taking a bath together.

“After I took my eyes off him, (Ryutaro) was face-down in the bathtub and floating unconscious,” police quoted Watanabe as saying.

When the ambulance crew arrived at the residence, the boy’s entire body was wet, according to sources close to the investigation.

The autopsy found water in the boy’s lungs but not enough to cause drowning.

The investigation also found that the boy’s internal organs were damaged and bleeding, leading police to believe the cause of death was internal blood loss.

Police suspect Watanabe assaulted the boy, then put him in the bathtub to make it look like a drowning death. The water in the boy’s lungs indicate he was breathing weakly when he was placed in the tub, the sources said.

Ryutaro’s mother was not at the residence when the incident occurred.