Charlie Savage reports today that the NSA doesn’t just monitor communications between Americans and terrorist suspects overseas. It monitors every communication sent overseas, searching for keywords linked to foreigners already under surveillance:

The N.S.A. 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, like a little used e-mail address, according to a senior intelligence official.

….To conduct the surveillance, the N.S.A. 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, like other former and current government officials, spoke on condition of anonymity because of the sensitivity of the topic, said the N.S.A. makes a “clone of selected communication links” to gather the communications, but declined to specify details, like the volume of the data that passes through them.

….The official said that a computer searches the data for the identifying keywords or other “selectors” and stores those that match so that 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 American communications that were flagged by the program.