whodunit

dekkari, salapoliisitarina A novel or drama concerning a crime (usually a murder) in which a detective follows clues to determine the perpetrator.
1946, Josephina Niggli. Pointers on radio writing (Boston: The Writer, Inc.) page 25

Actually, an whodunit is a mystery story. The hero is generally a detective, although sometimes he is a gentleman (apparently a mild mannered worm) who sallies forth at night to become Public Enemy No. One of Crime. Otherwise the formula is exactly the same as that of the Cliff-hanger and follows the same rules.