Since
nuclear explosions are famed not only for their destructive potential but also for the huge amounts of
electromagnetic radiation they chuck around, I believe that any sort of nuclear explosion would have been detected immediately after the event. The
US has
satellites designed for the purpose of spotting nuclear launches and explosions, and I'm sure that the
U.S. Government would not hide the fact that a nuclear explosion had occurred in a friendly country for long.
Plus, the
EM pulse would be detectable thousands of
miles away.