"You jump back too far. It merely shows that the criminal did not enter by the safe. That he left that way is proved by the fact that he vanished from the study without using door or windows, and that he very evidently took Darwin's key with him."
"But—the combination?"
"The safe was open, for Darwin had just removed the will from it. Even if it had been closed, a clever man could find an excuse for making his victim open the safe. Once inside any combination of six letters would close the door effectually against intruders."
"I suppose you are right, but how did he get in then?"
"Darwin let him in himself, either through the window or the door. Most probably through the window, since you would have otherwise heard steps in the hall. Recall Orton's testimony. He went to the garage to follow the maid. When he returned he heard voices in the study."
"And when he went in at eleven-thirty, Philip Darwin was alone," I remarked with a smile.
"Yes, to be sure, Philip Darwin was alone," he repeated, crestfallen.