Last week, PBS aired the first half of United States of Secrets, a Frontline documentary covering the rise of US surveillance in the wake of September 11th, 2001. Produced by Michael Kirk (who worked on the award-winning NFL expose League of Denial), the first half contextualized last year's leaked documents through interviews with government officials, whistleblowers, and journalists. It didn't include new revelations, but it captured the mood of a decade, when the intelligence community was given the power to prevent further terrorist attacks at any cost. As the world would learn in 2013, the NSA used those broadly defined powers to store the phone records of millions of Americans, denying that it did so until faced with a leaked court order sent to Verizon. Tonight, the second half of the series will pick up that thread again, this time focusing on the NSA's relationship with Silicon Valley.

If there's a key difference between the two halves, it's information availability. Part One focused on the call record ("bulk collection") program, and some questions remain. What percentage of phone records, for example, are actually collected? But the issue has been widely debated in Congress and the courts, where a number of cases are up for consideration by the Supreme Court. United States of Secrets: Privacy Lost, produced by journalist Martin Smith, focuses instead on the NSA's murkier internet surveillance, starting with the data collection program first known as PRISM. Leaked slides suggest that the agency has gathered emails and other information from major tech companies like Apple, Google, and Microsoft, and that it's tapped directly into fiber-optic cables to find more data without their consent.

Neither of these possibilities are things that major companies, which depend on the trust of their users, want to dwell on publicly, and their lack of comment means that there's less material to work with. But it also underscores the documentary's larger point. The problem isn't simply that tech giants give information to intelligence agencies, or that the NSA has compromised their security. Smith's segment suggests that it's a more fundamental problem with the ad-supported web, which runs by collecting and selling user information. The clip above refers to a report that intelligence agencies had piggybacked on Google's own tracking cookies to follow targets, alleging that Google tried to retaliate after the article was published.

"The moment you allow people to look at the content of your communication for some advertising purpose is the moment that the government is gonna come along and say, 'If you're gonna let them listen in for advertising, why don't you let us listen in for anti-terrorism or for serious crimes?'" says University of California Berkeley lecturer and privacy expert Chris Hoofnagle in the documentary. "And it becomes very difficult for courts to say that the private sector can listen in but the government can't."

The second part of United States of Secrets airs tonight at 10pm on PBS and online. The first part can be watched online now.