I’ve always thought that the criminals would start shouting police or acting as police as a means to get the jump on someone. Shouting police is not enough for the police to identify themselves. Any criminal can do that.
Already been done... Research "The Hillside Strangler" (who incidently, wasn't one, but *two* men, cousins). They used a security officer's badge to stop unsuspecting young women, who were then sexually assaulted, tortured, and murdered...
the infowarrior