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.