His mind began to move at length. The more he thought of it the more
absolutely certain he was that he had fastened the door before leaving
the house. True, the latch was only an ordinary one, and a key might
easily have been made to fit it. As a matter of fact, David had two, one
in reserve in case of accidents. The other was usually kept in a
jewel-drawer of the dressing-table. Perhaps--

David went quietly upstairs. It was just possible that the murderer was
in the house. But the closest search brought nothing to light. He pulled
out the jewel-drawer in the dressing-table. The spare latchkey had gone!
Here was something to go upon.

Then there was a rumbling of an electric bell somewhere that set David's
heart beating like a drum. The hall light streamed on a policeman in
uniform and an inspector in a dark overcoat and a hard felt hat. On the
pavement was a long shallow tray, which David recognised mechanically as
the ambulance.

"Something very serious, sir?" Inspector Marley asked, quietly. "I've
brought the doctor with me."

David nodded. Both the inspector and the doctor were acquaintances of
his. He closed the door and led the way into the study. Just inside the
conservatory and not far from the huddled figure lay David's new
cigar-case. Doubtless, without knowing it, the owner had whisked it off
the table when he had sprung the telephone.

"'Um," Marley muttered. "Is this a clue, or yours, sir?"

He lifted the case with its diamonds gleaming like stars on a dark night.