PIERRE THOMAS ( @PierreTABC), MIKE LEVINE ( @mlevinereports), JACK DATE ( @jackdate), LUIS MARTINEZ ( @LMartinezABC) and JACK CLOHERTY ( @jjclo) report:

As the U.S. intelligence community struggles to complete a damage assessment over the secret information allegedly stolen by NSA leaker Edward Snowden, sources told ABC News there is a growing consensus within the top circles of the U.S. government that the 30-year-old contractor could deal a potentially devastating blow to U.S. national security.

RELATED: In Their Own Words: Edward Snowden a Hero or Traitor?

Several officials warned the amount of compromised material may be much broader than even Snowden has suggested and that officials are not sure they know everything he may have pilfered. Another official said even the damage assessment won't be finished for some time.

Among the chief concerns, according to those officials:

Technical Roadmap of the U.S. Surveillance Network

Before he fled Hawaii for Hong Kong in late May, Snowden allegedly downloaded significant amounts of information about some of the country's most sensitive secrets - specifically how the U.S. government does surveillance abroad. One source told ABC News that as an information specialist with security clearance "he understood the framework of how the whole U.S. surveillance network works."

In short, Snowden's stolen material would help America's adversaries understand how we use electronics to spy.

Another official said Snowden had access to a particularly important computer server in the government's system "which contained ridiculous amounts of information" totaling hundreds of pages worth of secrets. He is suspected of storing stolen material on computers and making copies of documents. At risk is the effectiveness of billions of dollars worth of supercomputer and clandestine spying resources.

What Snowden May Know About Human Ops

Beyond technical systems, U.S. officials are deeply concerned that Snowden used his sensitive position to read about U.S. human assets, for example spies and informants overseas as well as safe houses and key spying centers.

They worry this recent quote from Snowden was not an exaggeration: " I had access to the full rosters of everyone working at the NSA, the entire intelligence community, and undercover assets all over the world. The locations of every station, we have what their missions are, and so forth."

So it's not just about what he took, but what he knows, officials emphasize. Officials describe Snowden as a walking treasure trove, a dream for foreign intelligence services. One intelligence official called Snowden and his cache an "entire U.S. government problem."

Known Damage Already

A senior intelligence official said: "The intelligence community is already seeing indications that several terrorist groups are in fact attempting to change their communication behaviors based on what they're reading about our surveillance programs in the media."

In an interview with CNN today, Secretary of State John Kerry said that "people may die as a consequence of what this man [Snowden] did."

"It is possible that the United States will be attacked because terrorists may now know how to protect themselves in some way or another, that they didn't know before," he said.

How Urgently Does the U.S. Want Snowden Back?

Attorney General Eric Holder called his counterpart in Hong Kong last week to lobby for Snowden to be arrested and extradited. Hong Kong failed to apprehend Snowden, however, claiming today they had no legal basis to do so. Now the U.S. government is sternly calling on Russia to "examine all options available to them" to expel Snowden to the U.S.

How the System Failed, and How to Fix It

On "This Week With George Stephanopolous" Sunday, NSA chief Gen. Keith Alexander said the system to keep classified information from leaking out "did not work as it should have" and said he didn't understand why it failed.

Today James Clapper, the Director of National Security, is seeking "more specificity" about IT professionals like Snowden used by the government as contractors to the intelligence community. The DNI annually makes an inventory of contractors in the spy community, but "in view of recent unauthorized disclosures of classified information," Clapper is focusing on IT personnel, according to a DNI spokesperson.