“How did he persuade these people to come to their death?” asked the inspector in a voice little above a whisper.
“That is a question for the psychologist,” said Michael at last. “There is no doubt that he got into touch with many men who were contemplating suicide but shrank from the act, and performed this service for them. I should imagine his practice of leaving around their heads for identification arose out of some poor wretch’s desire that his wife and family should secure his insurance.
“He worked with extraordinary cunning. The letters, as you know, went to a house of call and were collected by an old woman, who posted them to a second address, whence they were put in prepared envelopes and posted, ostensibly to London. I discovered that the envelopes were kept in a specially light-proof box, and that the unknown advertiser had stipulated that they should not be taken out of that box until they were ready for posting. An hour after those letters were put in the mail the address faded and became invisible, and another appeared.”
“Vanishing ink?”
Mike nodded.
“It is a trick that criminals frequently employ. The new address, of course, was Dower House. Put out the lights and let us go up.”
Three lamps were extinguished, and the detective looked round fearfully at the shadows.
“I think we’ll leave this down here,” he said.
“I think we will,” said Michael, in complete agreement.