14th Deadly Sin by James Patterson,Maxine Paetro (Women's Murder Club #14)

Detective Lindsay Boxer and her three best friends are back and recovering from the events that pushed them all to the edge. After her near-death experience, Yuki is seeing her life from a new perspective and is considering a change in her law career. San Francisco Chronicle reporter Cindy has healed from her gunshot wound and has published a book on the infamous serial killers she helped to bring down. Lindsay is just happy that the gang are all still in one piece.
But a new terror is sweeping the streets of San Francisco. A gang dressed as cops are ransacking the city, and leaving a string of dead bodies in their wake. Lindsay is on the case to track them down and needs to discover whether these killers could actually be police officers. Maybe even cops she already knows...