The National Security Agency has built a surveillance system capable of recording “100 percent” of a foreign country’s telephone calls, enabling the agency to rewind and review conversations as long as a month after they take place, according to people with direct knowledge of the effort and documents supplied by former contractor Edward Snowden.

A senior manager for the program compares it to a time machine — one that can replay the voices from any call without requiring that a person be identified in advance for surveillance.

The voice interception program, called MYSTIC, began in 2009. Its RETRO tool, short for “retrospective retrieval,” and related projects reached full capacity against the first target nation in 2011. Planning documents two years later anticipated similar operations elsewhere.

In the initial deployment, collection systems are recording “every single” conversation nationwide, storing billions of them in a 30-day rolling buffer that clears the oldest calls as new ones arrive, according to a classified summary.

The call buffer opens a door “into the past,” the summary says, enabling users to “retrieve audio of interest that was not tasked at the time of the original call.” Analysts listen to only a fraction of 1 percent of the calls, but the absolute numbers are high. Each month, they send millions of voice clippings, or “cuts,” for processing and long-term storage.

At the request of U.S. officials, The Washington Post is withholding details that could be used to identify the country where the system is being employed or other countries where its use was envisioned.

No other NSA program disclosed to date has swallowed a nation’s telephone network whole. Outside experts have sometimes described that prospect as disquieting but remote, with notable implications for a growing debate over the NSA’s practice of “bulk collection” abroad.

Bulk methods capture massive data flows “without the use of discriminants,” as President Obama put it in January. By design, they vacuum up all the data they touch — meaning that most of the conversations collected by RETRO would be irrelevant to U.S. national security interests.

In the view of U.S. officials, however, the capability is highly valuable.

In a statement, Caitlin Hayden, spokeswoman for the National Security Council, declined to comment on “specific alleged intelligence activities.” Speaking generally, she said that “new or emerging threats” are “often hidden within the large and complex system of modern global communications, and the United States must consequently collect signals intelligence in bulk in certain circumstances in order to identify these threats.”

NSA spokeswoman Vanee Vines, in an e-mailed statement, said that “continuous and selective reporting of specific techniques and tools used for legitimate U.S. foreign intelligence activities is highly detrimental to the national security of the United States and of our allies, and places at risk those we are sworn to protect.”

Some of the documents provided by Snowden suggest that high-volume eavesdropping may soon be extended to other countries, if it has not been already. The RETRO tool was built three years ago as a “unique one-off capability,” but last year’s secret intelligence budgetnamed five more countries for which the MYSTIC program provides “comprehensive metadata access and content,” with a sixth expected to be in place by last October.

The budget did not say whether the NSA now records calls in quantity in those countries or expects to do so. A separate document placed a high priority on planning “for MYSTIC accesses against projected new mission requirements,” including “voice.”

Ubiquitous voice surveillance, even overseas, pulls in a great deal of content from Americans who telephone, visit and work in the target country. It may also be seen as inconsistent with Obama’s Jan. 17 pledge “that the United States is not spying on ordinary people who don’t threaten our national security,” regardless of nationality, “and that we take their privacy concerns into account.”

In a presidential policy directive, Obama instructed the NSA and other agencies that bulk acquisition may be used only to gather intelligence related to one of six specified threats, including nuclear proliferation and terrorism. The directive, however, also noted that limits on bulk collection “do not apply to signals intelligence data that is temporarily acquired to facilitate targeted collection.”

The emblem of the MYSTIC program depicts a cartoon wizard with a telephone-headed staff. Among the agency’s bulk collection programs disclosed over the past year, its focus on the spoken word is unique. Most of the programs have involved the bulk collection of metadata — which does not include call content — or text, such as email address books.

Telephone calls are often thought to be more ephemeral and less suited than text for processing, storage and search. And there are indications that the call-recording program has been hindered by the NSA’s limited capacity to store and transmit bulky voice files.

In the first year of its deployment, a program officer wrote that the project “has long since reached the point where it was collecting and sending home far more than the bandwidth could handle.”

Because of similar capacity limits across a range of collection programs, the NSA is leaping forward with cloud-based collection systems and a gargantuan new “mission data repository” in Utah. According to its overview briefing, the Utah facility is designed “to cope with the vast increases in digital data that have accompanied the rise of the global network.”

Christopher Soghoian, the principal technologist for the American Civil Liberties Union, said history suggests that “over the next couple of years they will expand to more countries, retain data longer and expand the secondary uses.”

Spokesmen for the NSA and the office of Director of National Intelligence James R. Clapper Jr. declined to confirm or deny expansion plans or discuss the criteria for any change.

Based on RETRO’s internal reviews, the NSA has a strong motive to deploy it elsewhere. In the documents and in interviews, U.S. officials said RETRO is uniquely valuable when an analyst uncovers a new name or telephone number of interest.

With up to 30 days of recorded conversations in hand, the NSA can pull an instant history of the subject’s movements, associates and plans. Some other U.S. intelligence agencies also have access to RETRO.

Highly classified briefings cite examples in which the tool offered high-stakes intelligence that would not have existed under traditional surveillance programs in which subjects are identified for targeting in advance. In contrast with most of the government’s public claims about the value of controversial programs, the briefings supply names, dates, locations and fragments of intercepted calls in convincing detail.

Present and former U.S. officials, speaking on the condition of anonymity to provide context for a classified program, acknowledged that large numbers of conversations involving Americans would be gathered from the country where RETRO operates.

The NSA does not attempt to filter out their calls, defining them as communications “acquired incidentally as a result of collection directed against appropriate foreign intelligence targets.”

Until about 20 years ago, such incidental collection was unusual unless an American was communicating directly with a foreign intelligence target. In bulk collection systems, which are exponentially more capable than the ones in use throughout the Cold War, calls and other data from U.S. citizens and permanent residents are regularly ingested by the millions.

Under the NSA’s internal “minimization rules,” those intercepted communications “may be retained and processed” and included in intelligence reports. The agency generally removes the names of U.S. callers, but there are several broadly worded exceptions.

An independent group tasked by the White House to review U.S. surveillance policies recommended that incidentally collected U.S. calls and e-mails — including those obtained overseas — should nearly always “be purged upon detection.” Obama did not accept that recommendation.

Vines, in her statement, said the NSA’s work is “strictly conducted under the rule of law.”

RETRO and MYSTIC are carried out under Executive Order 12333, the traditional grant of presidential authority to intelligence agencies for operations outside the United States.

Since August, Sen. Dianne Feinstein (D-Calif.), the chairman of the Senate Intelligence Committee, and others on that panel have been working on plans to assert a greater oversight role for intelligence-gathering abroad. Some legislators are considering whether Congress should also draft new laws to govern those operations.

Experts say there is not much legislation that governs overseas intelligence work.

“Much of the U.S. government’s intelligence collection is not regulated by any statute passed by Congress,” said Timothy H. Edgar, the former director of privacy and civil liberties on Obama’s national security staff. “There’s a lot of focus on the Foreign Intelligence Surveillance Act, which is understandable, but that’s only a slice of what the intelligence community does.”

All surveillance must be properly authorized for a legitimate intelligence purpose, he said, but that “still leaves a gap for activities that otherwise basically aren’t regulated by law, because they’re not covered by FISA.”

Beginning in 2007, Congress loosened 40-year-old restrictions on domestic surveillance because so much foreign data crossed U.S. territory. There were no comparable changes to protect the privacy of U.S. citizens and residents whose calls and e-mails now routinely cross international borders.

Vines noted that the NSA’s job is to “identify threats within the large and complex system of modern global communications,” in which ordinary people share fiber-optic cables with legitimate intelligence targets.

For Peter Swire, a member of the president’s review group, the fact that Americans and foreigners use the same devices, software and networks calls for greater care to safeguard Americans’ privacy.

“It’s important to have institutional protections so that advanced capabilities used overseas don’t get turned against our democracy at home,” he said.

Originally publsihed by The Washington Post