Edward Snowden has very sensitive “blueprints” detailing how the National Security Agency operates that would allow someone who read them to evade or even duplicate NSA surveillance, a journalist close to the intelligence leaker has said.

Glenn Greenwald, a columnist with The Guardian newspaper who closely communicates with Snowden and first reported on his intelligence leaks, told The Associated Press on Sunday that the former NSA systems analyst has “literally thousands of documents” that 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,” Greenwald said in Brazil, adding that the interview was taking place about four hours after his last interaction with Snowden.

Snowden emerged from weeks of hiding in a Moscow airport on Friday, and said he was willing to meet President Vladimir Putin’s condition that he stops leaking US secrets if it means Russia would give him asylum until he can move on to Latin America.

Greenwald told The AP that Snowden has insisted the information from those documents not be made public. The journalist said it “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.”

Despite their sensitivity, Greenwald said he didn’t think that disclosure of the documents would prove harmful to Americans or their national security.

“I think it would be harmful to the US government, as they perceive their own interests, if the details of those programs were revealed,” 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 previously said the documents have been encrypted to help ensure their safekeeping.

Greenwald, who has also co-authored a series of articles in Rio de Janeiro’s O Globo newspaper focusing on NSA actions in Latin America, 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.

Greenwald said 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 US 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, speaking at a hotel in Rio de Janeiro, where he’s lived for the past eight years.