3rd Degree by James Patterson

The Women's Murder Club returns in a shockingly suspenseful thriller. Plunging into a burning town house, Detective Lindsay Boxer discovers three dead bodies...and a mysterious message at the scene. When more corpses turn up, Lindsay asks her friends Claire Washburn of the medical examiner's office, Assistant D.A. Jill Bernhardt, and San Francisco Chronicle reporter Cindy Thomas to help her find a murderer who vows to kill every three days. Even more terrifying, he has targeted one of the four friends. Which one will it be?