“I’ve got some stuff you might be interested in.”

With that simple message, sent in December 2012 from former NSA contractor Edward Snowden to Guardian reporter Glenn Greenwald, the biggest leak of government secrets in history was set in motion.

Greenwald paid little attention at first. As he explains in the above excerpt from United States of Secrets, premiering tonight on FRONTLINE, “Ninety-nine percent of the time, it ends up that they’re crazy or delusional, or the story is just not very good.”

But the source didn’t let up. He sent follow-up messages to Greenwald and documentary filmmaker Laura Poitras, and also contacted Barton Gellman of The Washington Post. Six months later, Greenwald and Poitras were on a flight to Hong Kong to meet him.

Snowden was promising an unprecedented scoop, but it wasn’t until the two journalists were finally airborne, in a space where they felt secure from government eavesdropping, that the they could finally examine together the secret documents that had been shared with them.

“I didn’t sleep one second for the next 16 hours because the adrenaline made that impossible,” Greenwald remembers. “We essentially couldn’t believe what it was that we had. And that was really the first time, I think, I fully understood that this was going to be unlike any other story — really ever — in American journalism or politics.”

What they had, specifically, were thousands — “not dozens or hundreds, but many thousands” — of top secret files showing that for the first time under the Obama administration, the communications records of millions of U.S. citizens were being collected in bulk, regardless of whether they had any connection to potential terror suspects.

“We all knew that this was incredibly risky and uncertain, but the story had to be reported,” Greenwald says.

In United States of Secrets, FRONTLINE tells the dramatic inside story of how the U.S. government came to spy on millions of Americans — and the extraordinary lengths taken to keep the effort hidden. From 9/11 to the Edward Snowden revelations, the two-part investigation is the definitive history of the government’s controversial surveillance program.

United States of Secrets (Part One) premieres on-air and online tonight, starting at a special time, 9 p.m. EST (check local listings). Part Two premieres next Tuesday, May 20.