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The voices are coming from what are known as "numbers stations," and they've long been thought to be part of international espionage operations. In fact, the Russian spies recently captured here in the U.S. may have been getting orders from Moscow via a shortwave numbers station
Additional story at: NPR with on-demand audio
http://www.npr.org/templates/story/story.php?storyId=128586766&sc=emaf
(William Hassig)