In general, however, the design of nuclear weapons has been the most closely guarded secret until long after the secrets had been independently developed—or stolen—by all the major powers and a number of lesser ones. It is generally possible to trace US knowledge of foreign progress in nuclear weapons technology by reading the US Department of Energy document "Restricted Data Declassification Decisions—1946 to the Present" (although some nuclear weapons design data have been reclassified since concern about proliferation of nuclear weapons to ""n"th countries" increased in the late 1970s).