The massacre occured five years ago in the Reigo Island, on which Ayatsuji happened to be at the island at that time. He solved the crime and the seventeen people who planned the murder all died. Inspector Asukai happened to be present that time and Mizuki Tsujimura has close ties to the incident.  The 18th murderer who is the mastermind of the incident is still not found, the only information about the mastermind is that it was a male and lost the first part of his ring finger. He was not killed since he escaped when Ayatsuji came to the island. 
- Bungo Stray Dogs Gaiden: Ayatsuji Yukito VS. Kyōgoku Natsuhiko.
- Bungo Stray Dogs Gaiden Manga: Gaiden Chapter 4.