This photo provided by The Guardian Newspaper in London shows Edward Snowden, who worked as a contract employee at the National Security Agency, on Sunday, June 9, 2013, in Hong Kong. (Glenn Greenwald and Laura Poitras/AP)

A National Security Agency internal review of damage caused by the former contractor Edward Snowden has focused on a particular area of concern: the possibility that he gained access to sensitive files that outline espionage operations against Chinese leaders and other critical targets, according to people familiar with aspects of the assessment.

The possibility that intelligence about foreign targets might be made public has stirred anxiety about the potential to compromise the agency’s overseas collection efforts. U.S. officials fear that further revelations could disclose specific intelligence-gathering methods or enable foreign governments to deduce their own vulnerabilities.

“We’re deeply concerned,” said one senior intelligence official, who, like others interviewed for this article, was not authorized to speak on the record. “The more that this gets made public, the more capability we lose.”

Snowden was able to range across hundreds of thousands of pages of documents on NSA networks, said one former official briefed on the issue. Another intelligence official cautioned that, at this point in the investigation, he did not appear to have obtained “collected data,” or the raw intelligence that results from hacking and other collection operations.

“He got a lot,” the official continued, but it was “not even close to the lion’s share” of what the NSA is engaged in. Still, the official said, harm to the efforts “is a concern.”

Snowden, 30, burst into global prominence a month ago, when he revealed he had passed top-
secret documents to The Washington Post and British newspaper the Guardian about classified U.S. surveillance programs. News reports based on the documents highlighted the U.S. government’s reach into the phone and Internet records of foreigners and ordinary Americans.

Snowden has told journalists that he has no desire to publicize information that describes the technical specifications or blueprints for how the NSA has constructed its eavesdropping network. At the same time, the former contractor has archived encrypted documents with people around the world, according to Glenn Greenwald, a journalist at the Guardian.

Greenwald told the online Daily Beast that “if anything happens” to Snowden, “the stories will inevitably be published.”

That has prompted concerns among U.S. officials that the documents are or could soon be outside of Snowden’s control.

It is not clear how many documents Snowden has given to others or whether those who have received the documents have the same set. Greenwald has said he has thousands of documents, but the Guardian has withheld “the majority of things that he gave us pursuant not only to his own instruction, but to our duty as journalists.” The Post has not said how many documents it has.

Investigators have largely determined what Snowden, who most recently worked at an NSA network operations center in Hawaii, was able to review within the agency’s systems. Their focus is determining which files he might have taken.

The damage assessment, being conducted by the Office of the National Counterintelligence Executive, is ongoing, and officials there had no comment. NSA officials also had no comment.

See the inner workings of the NSA’s PRISM program

Intelligence officials have said they have seen signs that several terrorist groups are changing their means of communication based on information that has been published. But those claims are impossible to verify independently.

That the United States is spying on China would be no revelation to the Chinese, although Snowden’s disclosures might constrain the United States’ ability to criticize the Chinese for theft of U.S. corporate secrets to aid its own industries. Snowden told the South China Morning Post newspaper that the NSA targeted civilian facilities in Hong Kong and mainland China.

He has denied reports that he provided classified information to the Chinese or Russian governments. He first sought refuge in Hong Kong and then fled to Moscow, where he is believed to remain.

The United States is engaged in high-level talks with Chinese officials this week on strategic economic and security issues, including the theft of U.S. corporate secrets by Chinese hackers to benefit Chinese industry.

But the release of information on how the NSA has penetrated Chinese networks would be especially damaging. “It’s not in the interests of the United States for the Chinese to know exactly how we do it,” said a former intelligence official. “It’s sources and methods.”

U.S. officials also fear that some of the documents Snowden has turned over to journalists disclose NSA methods of hacking into overseas networks, and, if published, will lead targets in other countries — in the Middle East, Europe, East Asia and South Asia — to take new defensive actions.

The Snowden leaks have set off a round of hand-wringing within the intelligence community and among allies about the inability to protect sensitive information. “There is a lot of annoyance at the United States,” the former official said.

The NSA, along with the rest of the intelligence community, began to put more information in computer networks after a government commission criticized intelligence and law enforcement agencies for failing to share information that could have prevented the Sept. 11, 2001, terrorist attacks. Restrictions on access based on an individual’s “need to know” gave way to the presumption of a “need to share.”

But that also made it easier for systems administrators such as Snowden, whose job was to make sure the networks worked properly, to gain access to files.

The NSA director, Gen. Keith Alexander, has testified that the agency is instituting a “two-
person” rule for oversight of systems administrators, to remove their ability to act unilaterally to gain access or make changes to restricted networks. It is similar to a rule created by the Pentagon after Army Pfc. Bradley Manning leaked hundreds of thousands of documents to the anti-secrecy group WikiLeaks.

Julie Tate contributed to this report.