Love Letter of a Murderer (1972)

A man (José Luis López Vázquez) walks into a bar and shoots four people dead. Next day everybody is talking about the murders. The police know who the killer is and have his photo, but the man himself remains at large. Blanca (Serena Vergano), a lonely librarian, receives a letter from him, declaring his love for her. Apparently he has been a regular at the municipal library, although Blanca denies ever having noticed him.