Thrill kill / Brian Thiem.

Enhanced descriptions from Syndetics:

Cops in Oakland seldom meet people whose lives are going well. That's certainly the case when homicide sergeant Matt Sinclair recognizes the dead woman hanging from a tree as a teenage runaway named Dawn he arrested ten years before. And as Sinclair and his partner, Cathy Braddock, soon learn, many of Dawn's clients, not to mention the local and federal officials who protect them, will go to any length to keep the police from digging too deep into her past.

Then the killer goes public, and Sinclair and Braddock must race to uncover the secrets Dawn was killed to protect before the killer unleashes a major attack on a scale the city has never seen before.

With Thrill Kill , Brian Thiem, a veteran of the Oakland Police Department and the Iraq war, has penned a nuanced police procedural that could only be written by a trained detective with years of hard-earned experience.