The Vigilantes (Badge of Honor Series #10)

Murders are on the rise in Philadelphia-but no one seems to mind because the victims are all fugitives with histories of heinous sex crimes against women and children. Worse for Homicide Sergeant Matt Payne, the main suspect is leaving evidence for police to find. But when copycat killings start popping up due to vigilante groups dealing out their own justice, Payne must find out who's behind the chaos before the violence overtakes the city.