Yakusoku: 16-Nen-me no Shinjitsu

When Kiriyuu Aoi was a child, her mother died, and her father then raised her alone in their suburban town. During the year 2008, a series of murders took place. One of the victims was the younger sister of a classmate that Kiriyuu Aoi liked. Kiriyuu Aoi was the person who found the body of her classmate's younger sister. Kiriyuu Aoi was so traumatized by the discovery that she lost some of her memories just before the incident. Making things worse, Kiriyuu Aoi’s father was arrested as the serial killer. With no one left around her, Kiriyuu Aoi left her hometown.
Now, 16 years later, Kiriyuu Aoi has become a detective. She returns to her hometown to prove her father's innocence, but another incident takes place. Detective Kiriyuu Aoi and her buddy detective Kosaka Kei chase after the truth behind the case. She soon learns the hidden faces of her ex-classmates from her youth.
(Source: AsianWiki)