Investigation of the case was at first entrusted to a detective named Fanferlot, nicknamed by his comrades the “Squirrel.”  Fanferlot’s examination of the premises resulted in little.  All he discovered was a scratch upon the door of the safe, but certain words that passed between M. Fauvel and his niece, which seemed to indicate that the former was secretly opposed to the marriage of Madeline with Bertomy, caused him to jump to the conclusion that the banker had robbed his own safe in order to bring disgrace upon his cashier.  He connived, however, at the arrest of Bertomy, hoping that later on he might obtain great kudos for himself by unmasking the banker.  What might have been the result of his improper and unofficial methods will never be known, but in all probability great inconvenience would have been caused to a number of innocent persons and the whole course of justice thwarted had it not been for the intervention of the great and famous M. Lecoq.

M. Lecoq’s interest in the bank robbery case was largely a personal one.  Even detectives have hearts, and M. Lecoq had loved with heart and soul a charming young girl named Nina Gipsy.  Under the name of Caldas in one of his innumerable disguises, he had wooed her for many months.  When he thought at last that he had won her affections, she had fled to the protection of no less a person than Prosper Bertomy himself.  The cashier cared nothing for her, but embittered by an estrangement that had sprung up between Madeline and himself, he had sought forgetfulness in her society.  Bertomy’s arrest gave Lecoq an opportunity for a noble revenge.  He determined to prove to the woman he loved his superiority over his rival by saving the cashier from disgrace.

Though the case looked black against Bertomy, for it was shown that he was heavily in debt, and living far beyond his means, Lecoq was satisfied that he had not committed the crime.  When Fanferlot, hopelessly befogged, called for his advice at his house in the Rue Montmartre, the great detective deigned to explain the preliminary data and the deductions from the data he had made.

The scratch on the safe door, slight and minute as it was, was his starting-point.  How had it been made?  He had found by experiment that it was impossible to make such a scratch upon the varnish without the exercise of considerable force.  It was clear, therefore, that the scratch by the keyhole could not have been made by the thief in his trembling anxiety to get the business he had undertaken accomplished.  But why was such force used?

For a long time Lecoq puzzled over this problem.  Then, with Fanferlot, he tried an experiment.  In his room was an iron box varnished like the safe.  Taking the key of this box from his pocket, he ordered Fanferlot to seize his arm just as he put it near the lock.  The key slipped, pulled away from the lock, and sliding along the surface of the door, left upon it a diagonal scratch, almost an exact reproduction of the one on the safe.