Glenn Greenwald, a columnist with the Guardian newspaper who first reported on the intelligence leaks, told The Associated Press that disclosure of the information in the documents "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 said the "literally thousands of documents" taken by Snowden constitute "basically the instruction manual for how the NSA is built."
"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," the journalist said Sunday in a Rio de Janeiro hotel room. He said the interview was taking place about four hours after his last interaction with Snowden.
Greenwald said he believes the disclosure of the information in the documents would not prove harmful to Americans or their national security, but that Snowden has insisted they not be made public.
"I think it would be harmful to the U.S. government, as they perceive their own interests, if the details of those programs were revealed," he said.
He has previously said the documents have been encrypted to help ensure their safekeeping.
Snowden emerged from weeks of hiding in a Moscow airport Friday, and said he was willing to meet President Vladimir Putin's condition that he stop leaking U.S. secrets if it means Russia would give him asylum until he can move on to Latin America.
Greenwald told The AP that he deliberately avoids talking to Snowden about issues related to where the former analyst might seek asylum in order to avoid possible legal problems for himself.
Snowden is believed to be stuck in the transit area of Moscow's main international airport, where he arrived from Hong Kong on June 23. He's had offers of asylum from Venezuela, Nicaragua and Bolivia, but because his U.S. passport has been revoked, the logistics of reaching whichever country he chooses are complicated.
Still, Greenwald said that Snowden remains "calm and tranquil," despite his predicament.
"I haven't sensed an iota of remorse or regret or anxiety over the situation that he's in," said Greenwald, who has lived in Brazil for the past eight years. "He's of course tense and focused on his security and his short-term well-being to the best extent that he can, but he's very resigned to the fact that things might go terribly wrong and he's at peace with that."
Greenwald said he worried that interest in Snowden's personal saga had detracted from the impact of his revelations, adding that Snowden deliberately turned down nearly all requests for interviews to avoid the media spotlight.
Asked whether Snowden seemed worried about his personal safety, Greenwald responded, "he's concerned."
He said the U.S. has shown it's "willing to take even the most extreme steps if they think doing so is necessary to neutralize a national security threat," Greenwald said. "He's aware of all those things, he's concerned about them but he's not going to be in any way paralyzed or constrained in what he thinks he can do as a result of that."
Asked about a so-called dead man's pact, which Greenwald has said would allow several people to access Snowden's trove of documents were anything to happen to him, Greenwald replied that "media descriptions of it have been overly simplistic.
"It's not just a matter of, if he dies, things get released, it's more nuanced than that," he said. "It's really just a way to protect himself against extremely rogue behaviour on the part of the United States, by which I mean violent actions toward him, designed to end his life, and it's just a way to ensure that nobody feels incentivized to do that."
He declined to provide any more details about the pact or how it would work.
Greenwald said he himself has beefed up his own security, particularly since a laptop went missing from his Rio home.
"I don't really feel comfortable discussing the specific measures, but one would be really irrational and foolish to have thousands of top-secret documents from the most secretive agency of the world's most powerful government and not be thoughtful about added security," said the 46-year-old former constitutional and civil rights lawyer who has written three books contending the government has violated personal rights in the name of protecting national security.
Greenwald has also co-authored a series of articles in Rio de Janeiro's O Globo newspaper focusing on NSA actions in Latin America. He said he expected to continue publishing further stories based on other Snowden documents over the next four months.
Upcoming stories would likely include details on "other domestic spying programs that have yet to be revealed," but which are similar in scope to those he has been reporting on. He did not provide further details on the nature of those programs.
It was not immediately clear whether Russia would take Snowden up on his latest request for asylum, which could further test U.S.-Russia relations.
Following Friday's meeting between Snowden and human rights activists, U.S. officials criticized Russia for allowing a "propaganda platform" for the NSA leader.
White House spokesman Jay Carney said Russia should instead send Snowden back to the U.S. to face the felony charges that are pending against him.
Carney said Snowden is not a human rights activist or a dissident. "He is accused of leaking classified information, has been charged with three felony counts and should be returned to the United States," the spokesman said.
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