"The man was mentally unsound," explained Marten. "The successful operation of his scheme excited his weakening brain until it became an obsession with him. Although he had achieved his original purpose of recuperating his broken fortunes, I believe he continued his threats and killing for the sheer insane joy of playing with his victim and then killing him. Possibly, the attendant mystery and notoriety also appealed to some perverted side of his nature."
"Go on with your story," said Forrester.
"These were the facts which I discovered about Prentice," continued Marten. "His mother died years ago in a private sanitarium for the insane on Long Island, New York. This showed a possibility that Prentice's mind might be affected, and in its weakness assume a criminal trend. I found that Prentice's father had willed him a certain amount of money, and that Prentice had lost approximately this amount in the stock market. That showed a possible reason for his step, for Prentice had a rather expensive wife and daughter to maintain, and he knew absolutely nothing of business. Prentice's father also willed him the piece of property upon which the oak tree stands. A few weeks before the affair started, Prentice ostensibly sold this property to a man named Hans Eckmeier, who in turn quickly sold it to a colored woman who lived in a small cottage on the property—a woman, by the way, without visible means of support, and without any financial resources which could be located. This unquestionably indicated preliminary preparation designed to eliminate any connection of Prentice with the property. There were rumors, too, that this woman had murdered her husband. Rumors are hardly to be considered as evidence, yet this story offered a possible basis for Prentice having a hold over the negress.
"Further investigation revealed the fact that Hans Eckmeier had received considerable financial assistance at different periods from Prentice, and was under obligation to him. Moreover, this man, who was a chemist, had invented a deadly poison gas, the formula of which he sold to the German Government just previous to the war. Here, Mr. Forrester, were a remarkable series of clues.
"If we could back these with a few pieces of actual evidence, our case was closed. That was the problem; how to get the evidence. I dared not appear in the case myself, nor have ordinary detectives come into contact with Prentice, without the danger of arousing his suspicions; yet it was imperative that someone possessing investigative instincts should come into close association with this man.
"During the war, while I was engaged in secret service work in New York, Miss Sturtevant, like many other women of her station in life, was of great assistance to the Government. Because of her social position it was possible for her to take her place in Prentice's social set without arousing any suspicion as to her purpose. It was arranged through the M.B.A. for her to rent the Bradbury house for the summer. This house was located sufficiently near the tree for her to watch it, and also within easy reach of Prentice's home. The M.B.A. arranged for Miss Sturtevant to receive letters of introduction to Chicago people who would be most helpful to her. The stage was completely set for our work just about the time you received your notice, and we hoped, through Prentice's attack upon you, to get the definite evidence we desired. That explains, for example, the telephone calls you received. I attempted to find out about the time you would place the money in the tree so as to make proper arrangements. When you refused the information Miss Sturtevant obtained it for me. We expected Prentice to go to the tree, and Miss Sturtevant was placed in a very fortunate position to watch him that night by being invited to a dinner and dance at his home; his family, of course, being entirely ignorant of his schemes. I received a report from her on Prentice's lawn, shortly after you left, and hurried to the tree. I was standing only a short distance back of the tree during the whole affair.
"You are familiar with the fiasco which the city detectives brought about that night. The package mysteriously disappeared, and as I learned later through Miss Sturtevant that Prentice left the dance for approximately three-quarters of an hour, he undoubtedly secured it. We were absolutely sure then that Prentice was the chief conspirator, probably being assisted by the chemist and the negress, Lucy. But we had no actual evidence.
"Our next opportunity was furnished by the Melville incident."
"Tell me one thing," interrupted Forrester. "What was Mary's connection with that affair?"
"I can explain that, Robert," replied the girl. "I knew that Mr. Melville was in danger when I saw Prentice there. After going to the conservatory with his son, it suddenly occurred to me that I might find some evidence in Prentice's car—particularly as I now knew about the gas and the mask through your adventure. Knowing that my next dances were with you, I felt that no special attention would be drawn to my absence, so I excused myself and slipped out. In the compartment under the front seat of Prentice's car I found one of the gas cylinders with the mask attached. Hoping to avert another murder I tried to stuff my handkerchief into the tube. The handkerchief was too large, so I tore it in half and then succeeded."