The thousands of people working on the Manhattan project stayed quiet.

There were probably only a few hundred people who were in a position to see every aspect of the program who would be able to see whether the whole thing would work. The rest could have been fooled.

Those who weren't would have been risking their lives and the lives of their whole families by talking. Also, If someone were to make a deathbed statement or someting, the press would just ignore it.

Here are some analyses of the American press.

ApolloHoax.net - The Lying American Press