Suddenly they all uttered an exclamation.  Something unusual, a rustling noise, had interrupted the silence.

M. Desmalions at once switched on the light.  He gave a cry.  A letter lay not on the table, but beside it, on the floor, on the carpet.

Mazeroux made the sign of the cross.  The inspectors were as pale as death.

M. Desmalions looked at Don Luis, who nodded his head without a word.

They inspected the condition of the locks and bolts.  Nothing had moved.

That day again, the contents of the letter made some amends for the really extraordinary manner of its delivery.  It completely dispelled all the doubts that still enshrouded the double murder on the Boulevard Suchet.

Again signed by the engineer, written throughout by himself, on the eighth of February, with no visible address, it said: 

“No, my dear friend, I will not allow myself to be killed like a sheep led to the slaughter.  I shall defend myself, I shall fight to the last moment.  Things have changed lately.  I have proofs now, undeniable proofs.  I possess letters that have passed between them.  And I know that they still love each other as they did at the start, that they want to marry, and that they will let nothing stand in their way.  It is written, understand what I say, it is written in Marie’s own hand; ’Have patience, my own Gaston.  My courage increases day by day.  So much the worse for him who stands between us.  He shall disappear.’

“My dear friend, if I succumb in the struggle you will find those letters (and all the evidence which I have collected against the wretched creature) in the safe hidden behind the small glass case:  Then revenge me.  Au revoir.  Perhaps good-bye.”

Thus ran the third missive.  Hippolyte Fauville from his grave named and accused his guilty wife.  From his grave he supplied the solution to the riddle and explained the reason why the crimes had been committed:  Marie Fauville and Gaston Sauverand were lovers.

Certainly they knew of the existence of Cosmo Mornington’s will, for they had begun by doing away with Cosmo Mornington; and their eagerness to come into the enormous fortune had hastened the catastrophe.  But the first idea of the murder rose from an older and deep-rooted passion:  Marie Fauville and Gaston Sauverand were lovers.

One problem remained to be solved:  who was the unknown correspondent to whom Hippolyte Fauville had bequeathed the task of avenging his murder, and who, instead of simply handing over the letters to the police, was exercising his ingenuity to deliver them by means of the most Machiavellian contrivances?  Was it to his interest also to remain in the background?

To all these questions Marie Fauville replied in the most unexpected manner, though it was one that fully accorded with her threats.  A week later, after a long cross-examination at which she was pressed for the name of her husband’s old friend and at which she maintained the most stubborn silence, together with a sort of stupid inertia, she returned to her cell in the evening and opened the veins of her wrist with a piece of glass which she had managed to hide.