When Rogers produced the collection of keys, Allison ran hurriedly over them and selected one which bore, on the handle, a small six-pointed star corresponding to the mark on the blue print.
"Want to go up with me and investigate the secret chamber?" he inquired.
"I certainly do," agreed Rogers. "But there's one point where this room won't help us in the slightest. How did the thief get the mail containing the money out of the building? You know the system that maintains in the registry room? It's practically impossible for a sheet of paper to be taken out of there, particularly when we are on guard, as we are now."
"That's true," Allison admitted, "but it's been my experience that problems which appear the most puzzling are, after all, the simplest of explanation. You remember the Philadelphia mint robbery—the one that Drummond solved in less than six hours? This may prove to be just as easy."
There Allison was wrong, dead wrong—as he had to admit some ten days later, when, worn with the strain of sitting for hours at a time with his eyes glued to the ventilator which masked the opening to the lookout, he finally came to the conclusion that something would have to be done to speed things up. It was true that no new robberies had occurred in the meantime, but neither had any of the old ones been punished. The lost one hundred thousand dollars was still lost; though the department, with the aid of the Treasury officials, had seen that the banks were reimbursed.
"The decoy letter," thought Allison, "is probably the oldest dodge in the world. But, who knows, it may work again in this case—provided we stage-manage it sufficiently carefully."
With the assistance of the cashier of one of the local banks Elmer arranged to have a dummy package of money forwarded by mail from New York. It was supposed to contain thirty-five thousand dollars in cash, and all the formalities were complied with precisely as if thirty-five thousand-dollar bills were really inside the envelope, instead of as many sheets of blank paper carefully arranged.
On the morning of the day the envelope was due to reach Columbus, Allison took up his position close to the grille in the lookout, his eyes strained to catch the slightest suspicious movement below. Hour after hour passed uneventfully until, almost immediately below him, he saw a man drop something on the floor. Two envelopes had slipped from his hands and he stooped to pick them up—that was all.
But what carried a thrill to the operative in the lookout was the fact that one of the envelopes was the dummy sent from New York and that, when the man straightened up, he had only one of the two in his hands. The dummy had disappeared!
Allison rubbed his eyes and looked again. No, he was right. The postal clerk had, in some manner, disposed of the envelope supposed to contain thirty-five thousand dollars and he was going about his work in precisely the same way as before.