Death in Big D by Dick Avery

Harry Calhoun, a private investigator with a fondness for scotch and scarred by his Army experience in Vietnam, is drawn into a search for a serial killer who is leaving victims all over Dallas, Texas. The victims have no connection with each other except they are all killed in the same manner: one shot between the eyes; another through the throat. As Harry doggedly tries to find the killer, the police threaten him with jail for interference in a police investigation. Does Harry succeed? Join him in the hunt and find out.