How the NSA Tapped AT&T’s Network

In 2002, an AT&T technician named Mark Klein discovered that the National Security Agency had installed a secret monitoring facility at the San Francisco building where he worked.

Four years later — after reading a New York Times story detailing the George W. Bush administration’s secret warrantless wiretapping program, Klein finally went public with what he’d seen.

Today, both the NSA and AT&T refuse to discuss what was going on in now-legendary Room 641A at 611 Folsom Street, as whatever was happening there remains classified.

In “United States of Secrets: Privacy Lost” — part two of FRONTLINE’s in-depth examination of the NSA’s secret surveillance programs — veteran correspondent Martin Smith explores the relationship between the U.S. surveillance state and America’s largest tech and telecom companies.

Part two premieres Tuesday, May 20 at 10 p.m. ET on PBS stations nationwide. FRONTLINE was kind enough to share an exclusive advance look with TIME.