> - The murderer was not in the room when the murder happened. Poison or a trap of some sort. (Kinda boring.) Slow-acting poison seems easily the most obvious

> - The murderer was not in the room when the murder happened. Poison or a trap of some sort. (Kinda boring.) Slow-acting poison seems easily the most obvious way to do it. I would hope every locked-room investigation quickly rules this out. > - The murderer was in the room when the murder happened, but locked the door behind themselves. (This one doesn't really work because the door is presumably of a kind that can't be locked behind you, or else no one would be surprised. Good to double-check, though, that you're sure the door couldn't have been locked behind someone leaving.) The clever trick might be in how the murderer managed to lock the door behind them. Using a magnet to shift the bolt, perhaps? (I'm sure this option must have been explored very early on in the locked-room genre.)