In her autobiography, Agatha Christie mentioned that she had to have a sort of deus ex machina (the message in the bottle, or whatever it was at the end) to tell the reader (and the police) who really did it and how it was done. Because, as the way it stood, how could you know? (Those who have read the book will understand what I mean; for those who have not read the book, I definitely don't want to spoil anything.)
