The Hollow Man - John Dickson Carr Part 1of 2 In to Thin Air
The story begins with the mysterious murder of Professor Grimaud, who is shot in his locked study. Witnesses hear the sound of his assailant's voice, but when the door is forced open, there's no sign of the attacker. Shortly after, another man linked to the case, Fley, is found dead in a snow-covered street, with no footprints leading to or from his body. Both murders appear impossible.