I'm writing a mystery story in which I want to make reference to a Agatha Christie story (I think it's a novel but I'm not sure). I can't remember the novel or even whether it was Poirot, Marple etc. All I know is that the characters all think they hear a gunshot in the library (or something), but it turns out the victim was killed much earlier and that the perp simulated the sound of the shot somehow (again, I don't remember details). I've been searching fan sites etc, but haven't found anything. Does any die-hard fan out there know which book this is?