Charles Frank describes keeping track of nuclear work being done in Germany during World War II.

Walker:

I'm particularly interested in the nuclear research.

Frank:

Nuclear stuff was not explicitly our problem. But we regarded it as our right and duty to keep a watching brief on anything that... But it was not an explicit general order to all intelligence sources, to send material about that to us. We had sufficiently good contacts with for example the code-breaking people, Frederick Norman, professor of German in London, in peacetime, was supposed to be our liaison man at Bletchley and I guess I talked to Norman on the scrambler phone at least once every day, and anything he thought would interest us, he saw came to us.

But virtually nothing that was explicitly nuclear. One or two things that one wondered what they were, that might be, came from that source, generally turned out not to be.

What we in fact were able to do I think fairly reliably in that class of evidence was, there couldn't in reality be a big German project which had got far enough to be actually thinking of building bombs, OK? Because if they had made it, if it had existed, well, it might not have had to use radio communication lines. If it did have to use radio communication, it would probably have been granted a special color, a special code block, different, so that when you'd broken another line of traffic, you're not into that one, and there were of course lines of German communication traffic which were not regularly read, I think there might be one or two that never were read, but I think there were, from their traffic habit I think we knew that those that we weren't reading, we knew roughly speaking what kind of thing, or really a lot of constraining factors, what kind of thing they might be dealing with.

So I think we were, on the other hand, one could have said of course, if they were making a nuclear bomb, it would be super-secret, and they might have made a ruling that they would never use any radio communication in connection with that, in which case our most reliable source of information would have been unavailable to us. So we were never able to really swear with absolute positive proof that the Germans were not making a bomb. We were only able to say, "It looks to us on all the evidence available that they're not."