Macabre Case of Prompiram

In 1977 a mentally retarded woman was traveling from Bangkok to Uttaradit near Chiang Mai by train without a ticket and was thrown off the train somewhere near Phitsanulok in central Thailand. The next day she was gang raped and murdered to shut her mouth... who was responsible for this? This film is about the police investigation into this case which is 100% true and was eventually solved.