The film was originally shot in the summer of 1977. However, the original version was considered unreleasable. Director Denny Harris turned to screenwriters Jim Wheat and Ken Wheat to help improve the story, but they had the radical idea to reshoot the bulk of the film. The actors and actresses who played the college students were brought back for reshoots in March 1978. Yvonne De Carlo, Barbara Steele, Cameron Mitchell, and Avery Schreiber were all brought in during reshoots for name-recognition value, replacing the actors who originally played their roles. In the end, only 12 minutes of footage from the original version was retained in the final finished film.
When Rebecca Balding was called back for reshoots 8 months after filming had concluded, she discovered they had discontinued the nail polish she'd worn in the original footage and had difficulty matching the color.
The Engels house is actually a Highland Park, Los Angeles home called The Smith Estate, and it was previously known as the Merrye house in the cult classic Spider Baby or, The Maddest Story Ever Told (1967). In both instances the house was chosen because it stands upon a hill, which made it easier to shoot wide angles and obscure the view of the neighboring homes in order to achieve a sense that the house is very isolated.
Director Denny Harris was a successful TV commercial director with his own self-named studio who dreamed of making his own horror movie. Utilizing his own staff, facilities (which doubled as the police station) and money, he churned out this independent film.