I knew that The Texas Chain Saw Massacre was, despite its claims, only tenuously related to any true-life story; I just didn’t realize how remotely:
The film, like the films Psycho, Deranged, and The Silence of the Lambs, was loosely inspired by Ed Gein. Gein did wear human skin, but he acted alone and did not use a chainsaw. Although the film’s opening would have one believe that the events are factual, it is merely a scare tactic, called the false document technique, to frighten the audience.
So it’s based on a single guy, not a family, in Wisconsin, not Texas, who did not use a chainsaw, and didn’t really commit a “massacre” either; he killed maybe two people, separately.