WASHINGTON -- The National Security Agency is searching the contents of vast amounts of Americans' email and text communications into and out of the country, hunting for people who mention information about foreigners under surveillance, according to intelligence officials.
The NSA is not just intercepting the communications of Americans who are in direct contact with foreigners targeted overseas, a practice that government officials have openly acknowledged. It is also casting a far wider net for people who cite information linked to those foreigners, such as a little-used email address, according to a senior intelligence official.
While it has long been known that the agency conducts extensive computer searches of data it vacuums up overseas, that it is systematically searching -- without warrants -- through the contents of Americans' communications that cross the border reveals more about the scale of its secret operations.
It also adds another element to the unfolding debate, provoked by the disclosures of former NSA contractor Edward J. Snowden, about whether the agency has infringed on Americans' privacy as it scoops up emails and phone data in its quest to ferret out foreign intelligence.
Government officials say the cross-border surveillance was authorized by a 2008 law, the FISA Amendments Act, in which Congress approved eavesdropping on domestic soil without warrants as long as the "target" was a noncitizen abroad. Voice communications are not included in that surveillance, the senior official said.
Asked to comment, NSA spokeswoman Judith A. Emmel did not directly address surveillance of cross-border communications. But she said the agency's activities were lawful and designed to gather intelligence not about Americans, but about "foreign powers and their agents, foreign organizations, foreign persons or international terrorists."
"In carrying out its signals intelligence mission, NSA collects only what it is explicitly authorized to collect," she said. "Moreover, the agency's activities are deployed only in response to requirements for information to protect the country and its interests."
Hints of the surveillance appeared in a set of rules, leaked by Mr. Snowden, for how the NSA may carry out the 2008 FISA law. One brief paragraph mentions that the agency "seeks to acquire communications about the target that are not to or from the target." The pages were posted online by The Guardian newspaper June 20, but the telltale paragraph -- the only rule marked "Top Secret" amid 18 pages of restrictions -- went largely overlooked amid a flurry of other disclosures.
To conduct the surveillance, the NSA is temporarily copying and then sifting through the contents of what is apparently most emails and other text-based communications that cross the border. The senior intelligence official, who, like other former and current government officials, spoke on condition of anonymity because of the topic's sensitivity, said the NSA makes a "clone of selected communication links" to gather the communications, but declined to specify details, such as the volume of data that passes through them.
Computer scientists said it would be difficult to systematically search the contents of the communications without first gathering nearly all cross-border text-based data; fiber-optic networks work by breaking messages into tiny packets that flow at the speed of light over different pathways to their shared destination, so they would need to be captured and reassembled.
The official said a computer searched the data for the identifying keywords or other "selectors" and stored those that match so human analysts could later examine them. The remaining communications, the official said, are deleted; the entire process takes "a small number of seconds," and the system has no ability to perform "retrospective searching."
The official said the keyword and other terms were "very precise" to minimize the number of innocent U.S. communications flagged by the program. At the same time, the official acknowledged that there had been times when changes by telecommunications providers or in the technology had led to inadvertent overcollection. The NSA monitors for these problems, fixes them and reports such incidents to its overseers in the government, the official said.
The disclosure sheds additional light on statements intelligence officials have made recently, reassuring the public that they do not "target" Americans for surveillance without warrants. At a House intelligence committee oversight hearing in June, for example, a lawmaker pressed NSA Deputy Director John Inglis to say whether the agency listened to the phone calls or read the emails and text messages of U.S. citizens. Mr. Inglis replied, "We do not target the content of U.S. person communications without a specific warrant anywhere on the earth."
Timothy Edgar, a former intelligence official in the Bush and Obama administrations, said the rule concerning collection "about" a person targeted for surveillance, rather than directed at that person, had provoked significant internal discussion. "There is an ambiguity in the law about what it means to 'target' someone," said Mr. Edgar, now a Brown University visiting professor. "You can never intentionally target someone inside the United States. Those are the words we were looking at. We were most concerned about making sure the procedures only target communications that have one party outside the United States."
The rule they ended up writing, secretly approved by the Foreign Intelligence Surveillance Court, says the NSA must ensure that one of the participants in any conversation acquired when it is searching for conversations about a targeted foreigner must be outside the United States, so the surveillance is technically directed at the foreign end.
Americans' communications singled out for further analysis are handled in accordance with "minimization" rules to protect privacy approved by the surveillance court. If private information is not relevant to understanding foreign intelligence, it is deleted; if it is relevant, the agency can retain it and disseminate it to other agencies, the rules show.
While the paragraph hinting at the surveillance has attracted little attention, the American Civil Liberties Union did take note of the "about the target" language in a June 21 post analyzing the larger set of rules, arguing that the language could be interpreted as allowing "bulk" collection of international communications, including of those of Americans. ACLU senior lawyer Jameel Jaffer said Wednesday that such "dragnet surveillance will be poisonous to the freedoms of inquiry and association," because people who know that their communications will be searched will change their behavior.