Kaasua, komisario Palmu! (1961)

A rich widow is found dead in her apartment in Helsinki due to gas poisoning. The incident is initially thought to be an accident, but inspector Palmu notices that the murderer made a mistake, and so Palmu, Kokki and Virta begin the investigation: who had come to the apartment through the balcony door and opened the gas tap while Mrs. Skrof was asleep?