"The pushpot motors explode," said Joe. "And they shouldn't. No bomb could be gotten into them without going off the instant they started, and they don't blow that way. I make a guess, sir, that one of the underground storage tanks--just one--contains doctored fuel. I'm guessing that as separate tanks in a pushpot are filled up, one by one, _one_ is filled from a particular underground storage tank that contains doctored fuel. The rest will have normal fuel. And the pushpot is going to crash when that tank, and only that tank, is used!" Major Holt was very silent. "You see, sir?" said Joe uneasily. "The pushpots could be fueled a hundred times over with perfectly good fuel, and then one tank in one of them would explode when drawn on. There'd be no pattern in the explosions...." Major Holt said coldly: "Of course I see! It would need only one tank of doctored fuel to be delivered to the airfield, and it need not be used for weeks. And there would be no trace in the wreckage, after the fire! You are telling me there is one underground storage tank in which the fuel is highly explosive. It is plausible. I will have it checked immediately." He hung up, and Joe went back to his meal. He felt uneasy. There couldn't be any way to make a jet motor explode unless you fed it explosive fuel. Then there couldn't be any way to stop it. And then--after the wreck had burned--there couldn't be any way to prove it was really sabotage. But the feeling of having reported only a guess was not too satisfying. Joe ate gloomily. He didn't pay much attention to Talley. He had that dogged, uncomfortable feeling a man has when he knows he doesn't qualify as an expert, but feels that he's hit on something the experts have missed. Half an hour after the evening mess--near sunset--a security officer wearing a uniform hunted up Joe at the airfield. "Major Holt sent me over to bring you back to the Shed," he said politely. "If you don't mind," said Joe with equal politeness, "I'll check that." He went to the phone booth in the barracks. He got Major Holt on the wire. And Major Holt hadn't sent anybody to get him. So Joe stayed in the telephone booth--on orders--while the Major did some fast telephoning. It was comforting to know he had a pistol in his pocket, and it was frustrating not to be allowed to try to capture the fake security officer himself. The idea of murdering Joe had not been given up, and he'd have liked to take part personally in protecting himself. But it was much more important for the fake security man to be captured than for Joe to have the satisfaction of attempting it himself. As a matter of fact, the fake officer started his getaway the instant Joe went to check on his orders. The officer knew they'd be found faked. It had not been practical for him to shoot Joe down where he was. There were too many people around for this murderer to have a chance at a getaway.