Powers of Darkness by Fred M. White

Grenfell examined the envelope carefully. He was not greatly excited at the discovery, for he had always known that he was the victim of a conspiracy, though the mystery of it was hard to unravel. Partridge had either been got rid of, or was a party to the fraud. It was, of course, possible he might have been robbed and murdered, but Grenfell never really thought so. Here was a striking proof of the correctness of his deductions.

But the envelope proved much more than the fact that Oliver Partridge was alive. It was conclusive evidence that he had himself been one of the conspirators. If not, why was he in communication with Draycott? Again, why had Draycott a part in the conspiracy? Perhaps Partridge, too, was blackmailing Draycott. Altogether, it was a tangle, and the more Grenfell dwelt upon it, the more puzzled he was.