Circle of Fear.

Michael Woods stars as Tony Giordani, a narcotics agent who learns that his wife has been murdered while he is recuperating from an attack by a mysterious stranger. His boss thinks it's a Mafia vendetta, but Tony follows the clues to an empty house which his wife had been photographing on the day of her murder. There, he finds a burned corpse and several clues leading to the Full Moon Killer, who has been decapitating prostitutes around the city. The owner of the house is an insane countess, whose escape from an asylum leads to yet another murder. Before long, Tony and his partner, Lisa, with whom he is engaged in a passionate affair, find themselves targeted for murder unless they can solve the case.