Gas, Inspector Palmu! (1961)

A rich widow is found dead of gas poisoning in her flat in Helsinki. First the death is thought to be an accident, but lieutenant Palmu notices that the murderer has made a mistake, and Palmu and his assistants Kokki and Virta start an investigation into who has broken into the apartment through the balcony door and opened the gas tap when Mrs. Skrof was sleeping.