The solution to a Locked Room Mystery can be one of several types, as typologised by John Dickson Carr in his novel The Hollow Man/The Three Coffins.
- (0.) There was a secret passage to the room or hidden hole in the wall: dismissed by Carr as an Ass Pull, which is probably justified unless there is some particularly plausible and unusual, but deducible, reason as to why the gap existed and was overlooked. Slightly less of an Ass Pull is the clearly visible hole in the wall which is wrongly dismissed because "nobody could possibly" have climbed up the chimney, or escaped through a tenth-story window.
- The victim died by accident in a way that appeared to be murder (often a Death by Falling Over).
- The victim was driven to kill himself by drugs, hypnotic suggestion, or blackmail.
- The murder was committed using some kind of hidden Booby Trap designed to give the impression of a human killer being present.
- Suicide, Not Murder.
- Tricks based around making the victim appear to be alive and active within the room when they were actually already dead (and, usually, killed by the last person to see them alive).
- Murder committed from a distance using a method designed to misleadingly suggest that the killer was at close range. A common version of this involves the use of melting, soluble, or otherwise decomposing projectiles to give the false impression that the murderer was inside the room and took the weapon away.note Killing at close range but from outside the room (for instance, using a narrow blade to stab them through the keyhole) is a variant.
- (6a.) Carr includes in 6 something which is arguably significant enough to deserve its own section, Time-Delayed Death where the fatal injury was inflicted outside the room and the victim sealed himself in either not noticing the injury or trying to get away from the killer. (This happens often enough in real life that if a murdered person is found in a room locked from the inside, the police will usually assume that this was the cause instead of worrying about it.)
- The reverse of 5., where the victim was killed immediately after the room was opened by one of the people who first "discovered" the situation. This would also include the solution used in a few stories, but not mentioned by Carr, of the murderer hiding inside the room until the body was discovered and hoping that in the shock of the discovery nobody would notice that they didn't enter the room with all the other horrified witnesses.
- The room was sealed from outside in such a way as to make it appear to have been sealed from the inside, or, similarly to 7., the room was never actually sealed and the first person to enter it falsely claimed or was tricked into believing that it was.
For people who are interested, the solution to the main murder in The Hollow Man is a (very complicated and unusual) combination of 5 and 6a.
