Computer scientists said that 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 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. 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 N.S.A. 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 the deputy director of the N.S.A., John Inglis, to say whether the agency listened to the phone calls or read the e-mails and text messages of American 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 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,” Mr. Edgar, now a visiting professor at Brown, said. “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.”