"There are two possibilities," Kennedy answered. "The real person behind the two murders may have employed some one else to carry out the actual killing, a hypothesis I do not take seriously, or"—again he paused—"this may be a case of some one with intelligence starting out upon his career of crime intelligently by reading up on his subject. It is as simple to learn how to use crotalin or botulin toxin or any number of hundreds of deadly substances as it is to obtain the majority of them. In fact, if people generally understood the ease with which whole communities could be wiped out, and grasped that it could be done so as to leave virtually no clue to the author of the horror, they might not sleep as soundly at night as they do. The saving grace is that the average criminal is often clever, but almost never truly scientific. Unfortunately, we have to combat one who possesses the latter quality to a high degree."
"What is the invisible menace of which you spoke, Craig?" I inquired.
"The possibility of another murder before we can apprehend the guilty person or gain the evidence we need."
"Good heavens!" I imagine I blanched. "You mean—"
"Werner was struck down, apparently, for no reason but that he had guessed the identity of the villain. There is a second man in the company who has certain suspicions and is acting upon them. If he is on the right trail, by any chance—" Kennedy shrugged his shoulders soberly.
"Shirley?"
"Exactly! And there is still another possibility."
"What is that?"
"Here in this laboratory I have blood spots made on the portieres at the house of Phelps by the man who removed the needle, probably the unknown himself, possibly his—or her—agent. In any case it is a clue and—THE ONLY DIRECT AND INFALLIBLE CLUE IN EXISTENCE TO THE CRIMINAL! Also I have the evidence of the snake venom and of the botulin toxin here. Sooner or later the person who killed Werner because he suspected things will wake up to the fact that we possess tangible proof against him."
I grew pale. "You mean, then, that you may be attacked yourself? That even I—"