When NSA contractor Edward Snowden downloaded tens of thousands of top-secret documents from a highly secure government network, it led to the largest leak of classified information in history - and sparked a fierce debate over privacy, technology and democracy in the post-9/11 world. Now, in United States Of Secrets," FRONTLINE goes behind the headlines to reveal the dramatic inside story of how the U.S. government came to monitor and collect the communications of millions of people around the world-including ordinary Americans-and the lengths they went to trying to hide the massive surveillance program from the public. "This is as close to the complete picture as anyone has yet put together - and it's bigger and more pervasive than we thought," says veteran FRONTLINE filmmaker Michael Kirk. In part one ... Kirk [pieces] together the secret history of the unprecedented surveillance program that began in the wake of September 11 and continues today - even after the revelations of its existence by Edward Snowden. Then, in part two, premiering Tuesday, May 20 ..., veteran FRONTLINE filmmaker Martin Smith continues the story, exploring the secret relationship between Silicon Valley and the National Security Agency, and investigating how the government and tech companies have worked together to gather and warehouse your data. "Through in-depth interviews with more than 60 whistleblowers, elected officials, journalists, intelligence insiders and cabinet officials, we have woven together the secret narrative that reveals the scale and scope of the government's spying program," says Kirk.