The New York Times reports today on a letter written by Danish physicist Niels Bohr in the 50s that recounts the specifics of a meeting in 1941 with Werner Heisenberg, the attempted father of the German nuclear weapon. The letter is seen as finally putting to rest Heisenberg's contention - examined in a book (which I happened to read) Heisenberg's War which turned into a play, Copenhagen. The letter to Heisenberg was amended several times but never sent.
Most of the people quoted in the story, with the exception of Heisenberg's grandson, see the letter as confirming that Heisenberg was a gung-ho bombmaker. The uncertainty still persists for me: if you were a German in 1941 meeting with a Dane, no matter how much your friendship and comradeship meant, would you have dared to suggest you were undermining the war effort? Or perhaps he hadn't evolved to that point yet.
The book makes it clear that regardless of whether Heisenberg and his team were purposely delaying the bomb, they simply did not have all the pieces together to make one, and that after their capture when they heard about the bombing of Japan, they were surprised when they found out some of the details. (They were being bugged in the house they were held in.)