NSA headquarters at Fort Meade, Md. First Look/Trevor Paglen If you want the complete picture surrounding Edward Snowden and the National Security Agency, PBS' Frontline documentary "United States of Secrets" is where you should start.

While journalist Glenn Greenwald has promised many more leaks to come from the ex-NSA contractor, this two-part series reveals context, interviews with many more whistleblowers in the national security state, and the back story that brought us to the front page stories of mass surveillance with the aid of Silicon Valley heavyweights.

"This is as close to the complete picture as anyone has yet put together," veteran Frontline filmmaker Michael Kirk said in a statement. "And it’s bigger and more pervasive than we thought."

Part one, which aired on May 13, detailed "the program" that emerged shortly after Sept. 11, with whistleblowers describing a surveillance operation that turned from foreign targets into a domestic dragnet.

In part two, which Business Insider screened prior to tonight's airing on PBS, the series explores the secret relationship between the NSA and Silicon Valley tech companies, and how they have often worked in tandem to gather and warehouse personal data.

It's a thrilling and disturbing documentary, featuring interviews with a remarkable amount of the key players. You should definitely watch it, as these three anecdotes from part two of the series are just a small piece of the overall picture:

In 2003, an AT&T technician discovered a secret room being used to copy all internet traffic coming through his building

In one interview, AT&T technician Mark Klein talks about the mysterious Room 641a he found in his workplace in San Francisco. "There's no door handle, so it looks kind of odd," he said.

Inside the room, he found what appeared to be the government using a splitter to copy all internet traffic moving through the AT&T internet backbone.

"One half is going to the secret room, and the other half is going to its normal, assigned destination," Klein said. "But it's been copied in the process."

While he went public with his find after reading a New York Times report on the NSA's warrantless wiretapping program in 2005, the existence of the program has still never been confirmed nor denied. "It still remains an open question," one woman says.

That open question leads to an interesting exchange between the filmmaker and an agitated Robert Deitz, the general counsel for the NSA from 1998 to 2006, who refuses to comment.

One tech CEO challenged the constitutionality of the government demanding his customer data — and won

In the post-PATRIOT Act era, national security letters (NSL) demanding user information from tech companies have become an all-too-common practice, with most companies complying with the letters and never speaking out against them.

But Nick Merrill, CEO of a web hosting company called Calyx, decided to challenge the constitutionality of one such letter, which he received in 2004. Noting that it wasn't signed by a judge, Merrill said the letter asked for significant information from his company and instructed him that he couldn't tell anyone he had received it.

"I thought that it was not legal and not constitutional," Merril said. "I decided to disobey the commandment that I not tell anyone and I called my lawyer."

He was the first and only one in the tech world up until 2013 (a group of Connecticut librarians did so as well) to challenge NSL's on constitutional grounds. The FBI later withdrew the letter after an appeals court ruled it unconstitutional.

"I think that they were afraid that we would make it to the Supreme Court and they were not 100% certain that they would get the answer that they want," Merrill said.

But perhaps the most interesting exchange on the topic of NSL's is with Andrew McLaughlin, Google's Director of Public Policy from 2004 to 2009.

“I think that Nick challenged that is fantastic. I would love it if Google had challenged that," he says, before the interviewer asks him, "Why didn’t they?" He responds: "I have no clue. No idea."

Edward Snowden went ballistic after a Guardian reporter pulled out his iPhone to interview him at his Hong Kong hotel

With reporters Glenn Greenwald and Laura Poitras already meeting a not-yet-revealed Edward Snowden in a Hong Kong hotel, The Guardian sent a senior correspondent to vet him and make sure he was legit.

"I asked 'do you mind if I tape the interview on an iPhone' and as soon as he saw the iPhone, it was like bringing out a microphone direct into the NSA," said reporter Ewen MacAskill. "He was totally appalled. He said 'get that out of the room as quickly as possible.'"

Snowden had many other ways to ensure he wasn't being surveilled or recorded, which the reporters — not yet familiar with the massive treasure trove of documents eventually revealed — initially thought were odd.

"He would often put a blanket over his head when he wanted to enter his computer system to prevent overhead cameras from picking up the password to the encryption," said Glenn Greenwald.

As they examined some of the documents — with the first revealing the NSA had captured millions of phone records from Verizon customers on a daily basis — MacAskill sent a coded message back to his editors to indicate Snowden was the real deal: "The Guinness is good."

The Guardian only gave The White House four hours to respond. As it was clear the government was only trying to stall, they decided to go ahead with the story.

"Where we are now is in a place where we’re living behind one way mirrors," says Washington Post reporter Barton Gellman in the closing to the documentary. "Corporate America and law enforcement and national security state know so much about us. And we know so little about them. We know so little about what they’re doing. How they’re doing it. That is a very uncomfortable position to be in."

"United States of Secrets: Part 2" airs on PBS tonight at 10 p.m. Here's the trailer:

Correction: An earlier version of this article stated that Nick Merrill was the only person to challenge NSL's on constitutional grounds. This has been corrected to note he was the only one in the tech world, while also mentioning a group of Connecticut librarians who challenged them in 2005.