The N.S.A. also programmed computers to map the connections between telephone numbers in the United States and suspect numbers abroad, sometimes focussing on a geographic area, rather than on a specific person--for example, a region of Pakistan. Such calls often triggered a process, known as "chaining," in which subsequent calls to and from the American number were monitored and linked. The way it worked, one high-level Bush Administration intelligence official told me, was for the agency "to take the first number out to two, three, or more levels of separation, and see if one of them comes back"--if, say, someone down the chain was also calling the original, suspect number. As the chain grew longer, more and more Americans inevitably were drawn in.

As to the legality:

"It's the unresolved tension between the operators saying, 'Here's what we can build,' and the legal people saying, 'Just because you can build it doesn't mean you can use it.' " It's a tension that the President and his advisers have not even begun to come to terms with.