This movie was partly inspired by a high-profile murder case from October 2017, in which the dismembered remains of 9 people, known as the 'Zama 9 Murder Case', were discovered stashed in coolers in the apartment of Takahiro Shiraishi, also known as the 'Twitter Killer'.
The director Shinzo Katayama wrote the scene, where Satoshi was telling his daughter he recognised the killer on the train, based on his own father's telling of recognising a man wanted for murder while riding on a train, when Shinzo was in high school. The real life killer was caught a few years after and Shinzo realised his father was telling the truth.