According to John Dickson Carr, there are no less than seven distinct types of locked room mystery. At least that is what his character Dr Fell tells the readers in chapter seventeen of his 1935 classic The Hollow Man (widely regarded as one of the masterpieces of that genre). Having mentioned this last week, I have been prevailed upon to summarise Dr Fell’s categorisations as a reminder for those who have read the book some time ago or to enable those who have yet to do so to benefit from Dr Fell’s elucidation while skipping over the relevant chapter (as Dickson Carr invites them to do) so they can get on with the plot.
1 The murder is not a murder but is, in fact, an accident. The circumstances are such that it appears there has been a murder but this is not the case. Instead there has been a fatal accident within the locked room. Hence no murderer was present or has made his escape without leaving any trace. Often the accident will involve a fall with the victim striking their head a blow on the fender. This method is becoming more difficult to carry off now with open fireplaces being replaced by central heating.
2 The murder is achieved by means of a poison gas which overcomes the victim (perhaps driving him into a frenzied paroxysm which causes damage to the furniture leading to investigators mistakenly believing a desperate fight has taken place between the victim and the murderer).
3 The murder is done by a mechanical trap planted in the room, which is set off by the victim while the murderer is safely elsewhere. The trick here is to make the method by which the trap is sprung undetectable and, if the trap is concealed, for the weapon to return to (or else remain in) its place of concealment after being triggered – such as a gun hidden in the workings of a clock which fires when the clock is being wound (a method which surely is falling out of fashion due to the inexorable rise of battery or mains electricity powered clocks).
4 It is suicide which is rigged up to look like murder, frequently with the intention of incriminating an innocent party against whom the suicide holds a grudge. The weapon might be an icicle with which the victim stabs himself. The icicle then melts, speeded no doubt by the fading body heat of the “victim”. In the absence of a weapon in the body, murder is presumed with the supposed murderer having made his escape with the the weapon.
5 The murderer impersonates the victim after having first killed him. The murderer is later observed to enter the room disguised as the victim. He emerges immediately afterward having slipped out of the disguise thereby giving himself the alibi of having been seen to leave the room without having had sufficient time to commit the murder. The timing of the death is critical in this category; the elapsed time between actual and supposed later time of death must be sufficiently short for the body to be in an appropriate state of rigor mortis and at the correct post-mortem temperature.
6 The murderer manages to carry out the murder from outside the room in a manner which suggests that the murderer was inside the room to carry out the killing. Bullets made of ice, or even frozen blood, have been fired in through windows and subsequently melted leaving no trace to detect the method used.
7 This is the reverse of category 5. Here, the murderer has merely stunned or otherwise rendered the victim unconscious. They leave the room and after a suitable interval, during which no-one enters or leaves the now locked room, ensure they are on hand as efforts to break down the door are made. They ensure they are first into the room and in the initial confusion after entry is gained, they killed the unconscious victim swiftly – a stiletto is a favoured method – while misdirecting the others who have crashed into the room with them. This gives the impression that the victim has been lying dead in the hitherto locked room for some time.
If any of you are currently reading a book where it turns out that one of these tricks has been used, please don’t blame me for having outlined the method here. I’m only repeating Dr Fell’s 80 year old lecture which anyone can read for themselves if they follow the suggested reading recommended by our conference speakers at: