Maigret hésite

When a series of letters, written on expensive stationery, arrive at Maigret's desk stating that a murder will take place but that the writer is unsure as to who will die, who will do the killing, and when the killing will occur, Maigret's interest is piqued and he soon tracks the stationery down to the house of Emile Parendon, an eminent lawyer. But, once there, tracking down clues to a crime not yet committed is not so easy and when a murder does take place the choice of victim surprises even Maigret.