In a last hopeless endeavor to solve the mystery, Carver called in an expert to inspect the ventilator. It was made of steel, a quarter of an inch thick, and fastened into the door itself. There were no screws with which it could have been taken out and even if it had been removed, only the tiniest of mortals could have crept through. “Still,” said Carver, “if we could suppose that the ventilator was removable, we might have taken a leaf from Edgar Allen Poe and thought seriously of a trained monkey being introduced.” “There is the theory of the duplicate key--” “Which I dismiss,” said Carver. “I am satisfied that no duplicate key was used. If a duplicate key had been procurable, Felling or Walters as you call him, would have found his way to it. He is the cleverest man in that business and he has lived on duplicate keys all his life. He must have known that it was impossible to gain admission by such a method or he wouldn’t have taken the trouble to make one. He is a specialist in that line of business, probably the finest locksmith of the underworld.” “Then you suggest that this key was used?” Tab pointed to the table. “I not only suggest it, but I would swear to it,” said Carver quietly. “Look!” He pulled the door open so that the light fell upon the outside keyhole. “Do you see the little blood-spots?” he asked. “That key has not only been used from the outside, where it has left unmistakable markings, but the same has happened on the inside of the door.” He swung the door again and Tab saw the tell-tale stains. “That door was unlocked from the inside after the old man was dead and locked again upon him.” “But how did the key get back to the table?” asked the bewildered reporter. Mr. Carver shook his head. “A medical student was once asked by a professor whether Adam was ever a baby, and he replied: ‘God knows’--that is my answer to you!” he said. “We will leave the other boxes until tomorrow, Tab.” Carver led the way out of the vault, locked the door with the duplicate key and put it in his pocket. “My brain is dead,” said Tab. And it was then that he saw the new pin. VII From where he stood the light caught it and sent up a thread of silvery reflection. He stooped mechanically and picked it up. “What is that?” asked the detective curiously. “It looks to me like a pin,” said Tab. It was a very ordinary pin, silvery bright and about an inch and a half in length. In that sense it was of an unusual size, though it was the kind that is commonly used by bankers, who delight in fastening large documents together by this barbarous method. It was not straight; there was a slight bend in it, but otherwise it had not remarkable features. Tab looked at it stupidly.