IN THE SUMMER of 2013, I spent my days sifting through the most extensive archive of top-secret files that had ever reached the hands of an American journalist. In a spectacular act of transgression against the National Security Agency, where he worked as a contractor, Edward Snowden had transmitted tens of thousands of classified documents to me, the columnist Glenn Greenwald, and the documentary filmmaker Laura Poitras.

One of those documents, the first to be made public in June 2013, revealed that the NSA was tracking billions of telephone calls made by Americans inside the US. The program became notorious, but its full story has not been told.

The first accounts revealed only bare bones. If you placed a call, whether local or international, the NSA stored the number you dialed, as well as the date, time and duration of the call. It was domestic surveillance, plain and simple. When the story broke, the NSA discounted the intrusion on privacy. The agency collected "only metadata," it said, not the content of telephone calls. Only on rare occasions, it said, did it search the records for links among terrorists.

I decided to delve more deeply. The public debate was missing important information. It occurred to me that I did not even know what the records looked like. At first I imagined them in the form of a simple, if gargantuan, list. I assumed that the NSA cleaned up the list—date goes here, call duration there—and converted it to the agency's preferred "atomic sigint data format." Otherwise I thought of the records as inert. During a conversation at the Aspen Security Forum that July, six weeks after Snowden's first disclosure and three months after the Boston Marathon bombing, Admiral Dennis Blair, the former director of national intelligence, assured me that the records were "stored," untouched, until the next Boston bomber came along.