In 2013 Edward Snowden downloaded tens of thousands of top-secret documents from a highly secure government computer network. The revelations that followed touched off a fierce debate over the massive surveillance operations conducted by the National Security Agency.

Now FRONTLINE investigates the secret history of the unprecedented surveillance program that began in the wake of 9/11 and continues today. Through exclusive interviews with intelligence insiders, cabinet officials, and government whistle-blowers, Part One reveals how the U.S. government came to monitor the communications of millions of Americans and to collect billions of records on ordinary people around the world.

In Part Two of United States of Secrets, FRONTLINE explores the role of Silicon Valley in the National Security Agency’s dragnet. As big technology companies encouraged users to share more and more information about their lives, they created a trove of data that could be useful not simply to advertisers, but also to the government. The revelations of NSA contractor Edward Snowden would push Silicon Valley into the center of a debate over privacy and government surveillance.