The National Security Agency’s ability to spy on vast quantities of Internet traffic passing through the United States has relied on its extraordinary, decades-long partnership with a single company: the telecom giant AT&T. While it has been long known that American telecommunications companies worked closely with the spy agency, newly disclosed N.S.A. documents show that the relationship with AT&T has been considered unique and especially productive. One document described it as “highly collaborative,” while another lauded the company’s “extreme willingness to help.” AT&T’s cooperation has involved a broad range of classified activities, according to the documents, which date from 2003...