The Toilets Were Closed from the Inside (1976)

Original title: Les vécés étaient fermés de l'intérieur

A bus conductor gets dressed for work in the morning, goes to the toilet, where he is killed by a bomb. The Commissioner and his fat, bumbling assistant, Inspector Charbonnier are put on the case. After interviewing friends, wives, colleagues, and spying on strangers who might be connected, our heroes trace the assassin down to a mental institution where, it seems, the murder victim has been an inmate for the last three years...