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. The N.S.A.'s top-secret budget in 2013 for the AT&T partnership was more than twice that of the next-largest such program, [and] the company installed surveillance equipment in at least 17 of its Internet hubs on American soil, far more than its similarly sized competitor, Verizon. After the terrorist attacks of Sept. 11, 2001, AT&T ... began turning over emails and phone calls "within days" after the warrantless surveillance began in October 2001. In 2011, AT&T began handing over 1.1 billion domestic cellphone calling records a day to the N.S.A. after "a push to get this flow operational prior to the 10th anniversary of 9/11," according to an internal agency newsletter. In a 2006 lawsuit, a retired AT&T technician named Mark Klein claimed that ... he had seen a secret room in a company building in San Francisco where the N.S.A. had installed equipment. Mr. Klein claimed that AT&T was providing the N.S.A. with access to Internet traffic that AT&T transmits for other telecom companies. Such cooperative arrangements, known in the industry as "peering," mean that communications from customers of other companies could end up on AT&T's network.