The Damned Thing (1975)
Doctor who lived at the edge of a swamp was found murdered with his neck slaughtered. Investigator, together with a jury made up of local farmers, comes into the doctor's cabin and tries to discover the cause of his death. The two sources may be of any help: a diary of the doctor, and the testimony of a journalist, the doctor's friend, who was with him at the moment of death. Based on the Ambrose Bierce novel with the same name. swamp, swamp monster, swamp thing, marsh, mystery killer
ETC.
