One of AMC’s investigations had Technical Intelligence Division follow up its belief that the saucers were developed from Nazi technology. By the end of the year a review of German wartime research had produced no evidence to support the idea, and it seemed increasingly unlikely that the Soviets would test new technology in US airspace. But two other bits of data had emerged from TID’s technical enquiries.
First, if the saucers performed the maneuvers that had been reported of them, then they had to be built from something exotic. No known material would have been able to withstand the velocities or stresses involved. Second, according to the USAF’s Aeromedical Laboratory, neither could human flesh. Opinion within TID began to swing toward the extraterrestrial hypothesis (ETH). On the unproven assumptions that UFO reports were accurate and the objects were real, its proponents reasoned that only a highly advanced otherworldly civilization could have produced such craft.
(UFO: The Government Files, Brookesmith)