Greenwald told the AP that if Snowden were to release those documents, they would "allow somebody who read them to know exactly how the NSA does what it does, which would in turn allow them to evade that surveillance or replicate it."
He added: "In order to take documents with him that proved that what he was saying was true, he had to take ones that included very sensitive, detailed blueprints of how the NSA does what they do."
Snowden is still stranded in Moscow, with the U.S. working hard to extradite him. As Mark reported Friday, Snowden said he was seeking temporary asylum in Russia. Bolivia, Nicaragua and Venezuela have all offered him asylum.
Copyright 2013 NPR. To see more, visit http://www.npr.org/.