Deep Water <Shinen>

For an unknown reason, 13-year-old Aki Kasuga seems to have killed her parents when she was found by her younger sister and detective Toru Takahira at the crime scene of lake Ezu near the Kumamoto city. As a minor, the girl joins a reintegration program and changed her name; from now on living as Sei Kamihara. Six years later, a new crime takes place in Ezu, and this time it's a little girl who dies. A week later, several species in the zoo are found dead. What is the common thread of all these mysteries in Kumamoto?