Logical puzzles and arguments The police have three suspects for the number of Mr. Boddy: Professor Plum, Colonel Mustard, and Mr. Green. Professor Plum, Colonel Mustard, and Green each declare that they did not kill Boddy. Mustard also states that he did not know Boddy and that he was out of town the day Boddy was killed. Green also states that he saw both Plum and Mustard with Boddy the day of the killing and that either Plum or Mustard must have killed him. Assume one of the three men is guilty and that innocent men do not lie. The statements of the guilty man may or may not be true. Determine who killed Mr. Boddy and prove it.

Since Green contradicts Mustard, one is lying and Plum is innocent. But I don't see any way to distinguish between Green and Mustard.