Mr. Mercedes (2017)
A demented killer taunts a retired police detective with a series of lurid letters and emails, forcing the ex cop to undertake a private, and potentially felonious, crusade to bring the killer to justice before he can strike again
L.A.P.D.: To Protect and to Serve (2001)
Two cops, a rookie and his partner, a veteran, are increasingly troubled by wild and illegal behavior of some of their colleagues At first they try to ignore it, but it soon proves to be impossible to ignore