X Y Z - A Detective Story (Unabridged)

Sometimes in the course of his experience, a detective, while engaged in ferreting out the mystery of one crime, runs inadvertently upon the clue to another. But rarely has this been done in a manner more unexpected or with attendant circumstances of greater interest than in the instance I am now about to relate. For some time the penetration of certain Washington officials had been baffled by the clever devices of a gang of counterfeiters who had inundated the western portion of Massachusetts with spurious Treasury notes.
Some of the best talent of the Secret Service had been expended upon the matter, but with no favorable result, when, one day, notice was received at Washington that a number of suspicious-looking letters, addressed to the simple initials, X. Y. Z., Brandon, Mass., were being daily forwarded through the mails of that region; and it being deemed possible that a clue had at last been offered to the mystery in hand, I was sent northward to investigate.