Edward Snowden has highly sensitive documents on how the US National Security Agency is structured and operates that could harm the US government, but has insisted that they not be made public, a journalist close to the NSA leaker said.
Glenn Greenwald, a columnist with The Guardian who first reported on the intelligence leaks, said 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". The "literally thousands of documents" taken by Snowden constituted "basically the instruction manual for how the NSA is built".
To take documents with him that proved what he was saying was true he had to take ones that included very sensitive, detailed blueprints of how the NSA does what it does, the journalist said.
Greenwald believed disclosure of the documents' information would not prove harmful to Americans or their national security, but that Snowden had insisted they not be made public.
"I think it would be harmful to the US government, as they perceive their own interests, if the details of those programmes 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 on Friday and said he was willing to meet President Vladimir Putin's condition that he stop leaking US secrets if it meant Russia would give him asylum until he could move to Latin America.
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 He has had offers of asylum from Venezuela, Nicaragua and Bolivia, but because his US passport has been revoked, the logistics of reaching whichever country he chooses are complicated.
Greenwald has 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.
Coming stories would probably include details on "other domestic spying programmes that have yet to be revealed", but which are similar in scope to those he has been reporting on.
Following Friday's meeting between Snowden and human rights activists, US officials criticised Russia for allowing the NSA leaker a "propaganda platform".
Meanwhile, Hong Kong legislators are expected to debate tomorrow a motion, to be moved by Ma Fung-kwok, urging the government to seek clarification from Washington of Snowden's disclosure of US government hacking of computer systems in Hong Kong and to express "strong dissatisfaction with the US government in this regard".