How the NSA skirts 'no US citizen' rule when eavesdropping

The US National Security Agency is searching the contents of vast amounts of Americans' e-mail 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 e-mail 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 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 Edward Snowden, the former NSA contractor, about whether the agency has infringed on Americans' privacy as it scoops up e-mails and phone data 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 non-citizen abroad. Voice communications are not included in that surveillance.

Judith Emmel, an NSA spokeswoman, said the agency's activities were lawful and designed to gather intelligence not about Americans but about "foreign powers and their agents, foreign organisations, foreign persons or international terrorists".

"In carrying out its signals intelligence mission, NSA collects only what it is explicitly authorised 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 Snowden, for how the NSA may carry out the FISA law. One brief paragraph mentions that the agency "seeks to acquire communications about the target that are not to or from the target."

To conduct the surveillance, the NSA is temporarily copying and then sifting through the contents of what is apparently most e-mails and other text-based communications that cross the border. The senior intelligence official, who spoke on the condition of anonymity because of the sensitivity of the topic, said the NSA makes a "clone of selected communication links" to gather the communications, but declined to specify details, such as the volume of the data that passes through them.

Timothy Edgar, a former intelligence official in the Bush and Obama administrations, said that 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 Edgar, who is now a visiting professor at Brown University. "You can never intentionally target someone inside the United States. We were most concerned about making sure the procedures only target communications that have one party outside the United States."