The Guardian was asked to return or destroy the files to "guarantee that very sensitive information that posed a risk to national security didn't get into the wrong hands."
Europe is furious about allegations that the U.S. government has been bugging European embassies and missions in apparent violation of a 1961 convention on diplomatic relations.
It remains unclear how the world's most famous refugee would be able to leave the transit zone of Sheremetyevo Airport, where he has been marooned without valid documents since he arrived from Hong Kong on June 23.
