General Groves explains (in Now It Can Be Told: The Story of the Manhattan Project), that in making his initial appraisal of the German atomic picture, Captain Horace K. Calvert knew it would take a combination of three requisite factors to make a bomb:
Those were: (1) a sufficient number of top nuclear scientists and technical assistants; (2) the basic fuel for a bomb—uranium, and possibly thorium, probably combined with uranium; and (3) laboratories to develop it and industrial means to make it.
He started working on the fuel problem first, for we were sure of Germany’s scientific and industrial ability to do the job. Thorium seemed out of the question, since it is mined chiefly in Brazil and India and, because of embargoes, Germany had been unable to import any since the war began, and had had only insignificant stocks on hand before the war. The basic fuel was thought to be uranium. Considering our own firsthand knowledge of the enormous industrial effort required to produce U-235, we were confident that we would have seen evidences of any such program had one existed. It seemed more likely that they would use plutonium. That they had enough to launch an atomic program seemed to be within the realm of possibility, for we knew there had been a large stockpile of refined uranium ore at Oolen, Belgium, a few miles outside Brussels, which originally had been the property of Union Miniere.
The only other possible supply of uranium was the mines at Joachimsthal, Czechoslovakia, which was not a particularly significant source. Most of this ore was shipped to a uranium plant outside Berlin, the Auer-Gesellschaft. British Intelligence kept in touch with the activities of these mines, and in July, 1944, Calvert’s group started periodic aerial surveillance over the entire mining area, studying the pictures in detail for new shafts and aboveground activity. Tailing piles from each mine were microscopically measured from one reconnaissance to the next. By knowing the general grade of the ore and measuring the piles, we could determine with some degree of accuracy the mine’s daily production. There were no signs of extraordinary activity.
It would have been imperative for Hitler to enlist the aid of all his top scientists. Allied Intelligence had established that many of them were working on the “V” weapon; particularly at Peenemiinde, but to our knowledge no nuclear physicists had been reported there. Calvert started a search for some fifty German nuclear scientists. He knew that there must be many young scientists who had come up since Hitler’s rise to power of whom we had no knowledge; however, if we could locate a few of the top people, they should lead us to the rest. All the present and back issues of the German physics journals were scrutinized. Foreign-born nuclear scientists in the United States, like Enrico Fermi, O. R. Frisch and Niels Bohr, as well as anti-Nazi professors and scientists in Switzerland, Sweden and other neutral countries, were questioned in detail to obtain any past or present information they might have on the whereabouts of the German scientists. The names of all German scientists were placed on watch lists with American and British intelligence agencies which were daily scanning German newspapers that had been smuggled out. Before long we had recent addresses for a majority of the scientists in whom we were interested.
The third main category of pre-D-Day investigation, laboratories and industrial plants, was studied in much the same way. Lists were compiled of all of the precious metal refineries, the physics laboratories, the handlers of uranium and thorium, manufacturers of centrifugal and reciprocating pumps, power plants and other such installations as were known to exist in the Axis countries. These were placed on a master list from which they were not removed until we had positive information that they were not engaged in, or supplying, an atomic program. All plants where work of an unknown nature was being conducted were checked through aerial reconnaissance, the underground, OSS and all the numerous intelligence agencies.