Police are investigating how a man was able to escape from police custody while being detained at a police box ("koban") in Tokyo.

The Shakujii police department said the man, who was being held on suspicion of groping a woman in a train car on the Seibu Shinjuku line, escaped from the "koban" opposite Kami-shakujii Station in Nerima Ward on June 18, TBS reported.

The police sergeant on duty, a man in his 50s, made the suspect wait in a back room at around 8 p.m. while he questioned the alleged victim. When he later went to question the man, the sergeant realized that the suspect had left the station by an unlocked back door, TBS reported.

According to the Tokyo Metropolitan Police Department, the sergeant had not yet taken the man's name, and as such, his identity is unknown. Before being left alone in the room, the suspect reportedly told the officer, "This has nothing to do with me."

There was no other officer on duty at the time.

© Japan Today