I Know You Are a Murderer... (1972)

Dana, a young woman of loose morals, dies at the hands of a mentally unstable young man, Pavel. In a fit of jealousy, he could not restrain himself and killed the girl with a blow to the head. The police find the corpse thanks to Dana's friend Eva. And it's Eva who becomes Pavel's second victim. The young man, afraid of being found out, attacks the girl and drowns her in the bathtub. Lieutenant Zeman, a criminal investigator, is put in charge of the double murder as his first case, assisted by his more experienced colleague Suchánek. However, the investigation initially goes nowhere. While Pavel's father tries to commit his mentally ill son to a mental institution, Pavel commits another murder out of fear. Neither he nor the investigators know that Pavel has been a mere tool in someone else's hands for some time.