Police guard a door where human rights groups were taken to meet former intelligence agency contractor Edward Snowden at Sheremetyevo airport July 12, 2013. REUTERS/Grigory Dukor Thousands of documents that NSA whistleblower/leaker Edward Snowden stole from the NSA constitute "the instruction manual for how the NSA is built," Guardian journalist Glenn Greenwald told the Associated Press.

Greenwald added that particular 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."

So now we know why Snowden could be Washington's "worst nightmare."

And in the sense that the 30-year-old ex-Booz Allen employee has "access to some of the U.S. government's most highly-classified secrets," he already is.

As we have previously reported, citing the book "Deep State: Inside the Government Secrecy Industry," the most closely held secrets by the U.S. "are what we know about everyone else's secrets and how we came to know them."

That type information is precisely what Snowden carried on four laptops while spending a month in China, and is presumably included in the 10,000 secret documents Greenwald is carrying around with him wherever he goes.

As Greenwald previously said, "Snowden has enough information to cause harm to the U.S. government in a single minute than any other person has ever had."

Greenwald has said that the documents have been highly-encrypted so that they don't leak, but that statement — from a constitutional lawyer who didn't have encryption software before communicating with Snowden — has not allayed concerns of intelligence officials and the original NSA whistleblower.

“That stuff is gone,” a former senior U.S. intelligence official who served in Russia told The Washington Post last month. “I guarantee the Chinese intelligence service got their hands on that right away. If they imaged the hard drives and then returned them to him, well, then the Russians have that stuff now.”

Greenwald and Snowden pushed back against assertions such as that one, but the threats of Chinese and Russian intelligence remotely lifting that data are real.

Last year China expert Kenneth G. Lieberthal told The New York Times that when he travels to that country, he doesn't bring his cellphone or laptop. Instead, he brings “loaner” devices, which he erases before he leaves the U.S. and wipes clean the minute he returns.

Mike Rogers (R-Mich.), who chairs of the House Intelligence Committee, also told the Times that he travels “electronically naked.”

Russ Tice, the original NSA whistleblower who recently claimed that the NSA wiretapped then-Senator Barack Obama in 2004, found it hard to believe that Snowden would carry physical data on him — because of how dumb that would be.

"It would be foolish," Tice told Business Insider. "If he went out to lunch, the Chinese authorities would be searching his hotel room … to try to see if he had any more physical goodies on the NSA. And if he did, he certainly would not have left Hong Kong with that information without the Hong Kong authorities making sure they got it from him."

Greenwald contends that "there was never any evidence that this was true," which is true — but it's also highly unlikely that Chinese or Russian intelligence would indicate if they copied the data.

Just like we don't know what Chinese and Russian officials learned from interviewing the NSA-trained hacker.

After all, the subject at hand is international espionage.

Then there is the argument that even if China got the NSA's secrets, the data itself is highly encrypted. That's most likely true, but the NSA's own supercomputers aim to crack the world's strongest encryption.

And China's top supercomputer is almost twice as powerful as any other in the world.

Ultimately no one knows who has copies of what Snowden took. Although one important point, as former intelligence analyst Joshua Foust notes, is that it appears Snowden is no longer in control of his situation.

That's why sources told Reuters that U.S. authorities are operating on a "worst case" assumption that all of the classified material in Snowden's possession has made its way to one or more adversary intelligence services.

If those countries found out what was in those files, it would be catastrophic for America's ability to spy and cause of far greater damage than Snowden ever intended.

Greenwald told the AP that Snowden insisted that the NSA "blueprints" not be made public. But it is not ridiculous to wonder if China and Russia could have bested the former CIA technician.

Nevertheless, simply by having the potential to leak the NSA's modus operandi, Snowden has the U.S. government very concerned.