In the outside office, a telephone rang. Penny heard Miss Arrow answer the call. Apparently, the secretary was unaware that Mr. Nichols had stepped from the office, for she said: "Just a minute, please. I will connect you with him." An instant later the telephone at Penny's elbow jangled. She took the receiver from its hook intending to explain to the caller that Mr. Nichols had left the office. Before she could speak, a cold, precise masculine voice came to her over the wire. "Just a little warning, Mr. Nichols!" the words clipped into her ear. "Lay off the Molberg gang or else----" Penny heard a receiver click. The wire had gone dead. CHAPTER IV The Molberg Gang Penny signaled frantically for the operator's attention. It seemed minutes before the telephone girl responded mechanically: "Number please." "I was disconnected with my party," Penny informed tensely. "See if you can trace the call. It is very important." "Just a minute please." There was another long wait, then the telephone operator informed Penny that the call could not be traced. It had been made from a pay station. Mr. Nichols entered the office just as Penny hung up the phone. "Anything wrong?" he asked quickly, noticing the expression on her face. Penny repeated the warning message. "Well, it looks as if I'm on the right trail," Mr. Nichols declared, not in the least disturbed. "I'd have preferred that the Molberg gang hadn't learned I was shadowing them, but such news travels fast through underground channels." "I'm afraid some of those dreadful men may harm you," Penny said anxiously. "Promise me you'll be careful." "I am always careful, my dear, but I refuse to go around wearing a bullet proof vest. This isn't the first warning telephone call I've received." "I suppose not," Penny sighed. "But I should think that if the members of the Molberg gang know you have been assigned to the case, it would be hard to secure evidence against them." "It won't be easy," the detective agreed. "However, I flatter myself that I have a few trained investigators whose activities will never be suspected." "You mean they mingle with underworld characters and try to gain their confidence?" "Yes, that's the usual plan. When I locate Rap Molberg I'll have him constantly shadowed." "I've never seen many of your assistants around the office," Penny remarked. "Naturally not," Mr. Nichols smiled. "If they came here to report, every crook in Belton City would be aware of it within an hour." "Then how do you keep in touch with your men?" Penny asked curiously. "There are a few secrets which I must keep to myself. Aren't you taking a rather sudden interest in my work, Penny?" "Perhaps I am. Since my car wheels were stolen I feel personally concerned in the case. I wish I could do something to help."