Thousands of people pass by the buildings each day and rarely give them a second glance, because their function is not publicly known. They are an integral part of one of the world's largest telecommunications networks – and they are also linked to a controversial National Security Agency surveillance program.

Atlanta, Chicago, Dallas, Los Angeles, New York City, San Francisco, Seattle, and Washington, D.C. In each of these cities, The Intercept has identified an AT&T facility containing networking equipment that transports large quantities of internet traffic across the United States and the world. A body of evidence – including classified NSA documents, public records, and interviews with several former AT&T employees – indicates that the buildings are central to an NSA spying initiative that has for years monitored billions of emails, phone calls, and online chats passing across U.S. territory.