The National Security Agency has been saying for two months that it knows every secret former government contractor Edward Snowden pilfered regarding its surveillance programs. Now, it's not so sure.

NBC News reports that the NSA is "overwhelmed" by the task of assessing the damage, and still doesn't know the full scope of the classified information that remains in the hands of Snowden, who is holed up in Moscow hoping to dodge espionage charges he faces back in the U.S.

The amount of information Snowden took remains a central mystery of his case, and a main source of worry for intelligence officials.

Glenn Greenwald, the journalist who has published most of Snowden's leaks in Britain's the Guardian newspaper, has claimed that Snowden has "blueprints" for the NSA's operations that are so detailed they would let others evade or replicate the agency's surveillance programs. U.S. intelligence officials have said, however, that they doubt Snowden really managed to get his hands on the NSA's "crown jewels."

So what other leaks can the world expect? NBC's sources said the best bet is that the documents Snowden has yet to release contain details on spying by American allies, including the U.K., Canada, Australia, and New Zealand. "If true, that might explain the recent U.K.-led crackdown on journalists and (their partners) who have had access to Snowden's leaked documents," says Taylor Berman at Gawker.

Berman is referring to the decision by British authorities to detain Greenwald's partner, David Miranda, at London's Heathrow airport as he returned home to Brazil following a trip to Germany. Police confiscated Miranda's laptop, flash drives, and cell phone in what Greenwald called an attempt to intimidate him.

Officials say the NSA's poor auditing capabilities and Snowden's skill at covering his tracks are making it difficult to tell what other new revelations he might make. Senate Intelligence Committee Chairwoman Dianne Feinstein (D-Calif.) said last month that she had been told Snowden took roughly 200 classified documents, but the latest estimates are in the thousands.

The irony is that while the U.S. doesn't know what Snowden knows, China and Russia just might, from the time Snowden spent hiding out in their respective countries.

Like most observers, Jeffrey Toobin at The New Yorker doesn't buy Snowden's insistence that officials in those countries didn't peek at his information trove. "That is a preposterous proposition," Toobin says. "Even assuming that Snowden believes he had control of his computers 24/7 (he never slept?), there is simply no way that China and Russia would pass up that kind of bounty."

That probably means, Toobin says, that the NSA will have to spend billions of dollars and countless hours reworking its procedures, because it might never be confident of what intelligence Snowden possesses.