Qui?

Mademoiselle Blanche Durand, in her sixties who lives on the mezzanine level of her property, is found dead, stabbed, inside her home. To enter, the judge had to ask for help from an operator to remove the grate from the small window that faces the street. The door of the flat has two locks and a chain placed inside. Whoever committed the crime could not get out of there or out of the window, since it was embedded in the wall.