When a source with intimate knowledge of the National Security Agency’s domestic surveillance activities decided to reveal government secrets, he turned to an accomplished, if little known, documentary filmmaker to reveal the program to the world.

Laura Poitras, 49, had the odd distinction of sharing a byline in The Washington Post and in London’s Guardian newspaper last week on two blockbuster stories. The first, in The Post, revealed a secret NSA program called PRISM that used data from some of the Internet’s biggest firms. The second, in the Guardian, was a video interview with the source of the information, a former NSA operative from Maryland named Edward Snowden.

Poitras worked with two old friends — the Guardian’s Glenn Greenwald and The Post’s Barton Gellman — to bring the stories to light.

Although Poitras’s name appeared atop The Post’s story, she is not part of the newspaper’s staff (both she and Gellman, a former staffer, worked under a contract with the paper). Indeed, few in The Post’s newsroom were familiar with Poitras’s work until the NSA story broke in The Post and Guardian late Thursday.

That is most likely because Poitras has made a name for herself largely among the small circle of nonfiction filmmakers, whose work often takes years to come to fruition. The winner of a Peabody Award and a MacArthur “genius” grant last year, Poitras is the director of three films about the war on terrorism. Among her films is a short work, “The Program,” completed last year and posted on the New York Times’s Web site; it featured an interview with William Binney, a retired NSA analyst who disclosed details of another secret domestic surveillance program called “Stellar Wind.”

Director Laura Poitras arrives at the International Documentary Association’s 26th annual awards ceremony at the Directors Guild Of America on December 3, 2010 in Los Angeles, California. (Alberto E. Rodriguez)

Gellman declined to speak Monday at length about his collaboration with Poitras. “All I can tell you is she is a documentary filmmaker whose work I admire and with whom I shared an affiliation at an NYU think tank,” he said. “I consider her a friend.”

In Greenwald’s case, Poitras wasn’t just a friend and a collaborator but also a onetime source and subject. In a column he wrote for Salon.com last year, Greenwald described the “harassment” Poitras has faced from government officials whenever she returned to the United States from reporting abroad. He praised her at the time, writing, “Poitras produces some of the best, bravest and most important filmmaking and journalism of the past decade, often exposing truths that are adverse to U.S. government policy, concerning the most sensitive and consequential matters. . . .”

It was Greenwald’s column on her, as well as “The Program,” that apparently prompted Snowden to contact her anonymously with revelations about PRISM, Poitras said in an interview with Salon late Monday.

Poitras, who could not be reached Monday by The Post, is in Hong Kong, where she recorded the Guardian’s video interview with Snowden. However, she told Salon that Snowden contacted her and Greenwald separately around the same time in February.

“He told me he’d contacted me because my border harassment meant that I’d been a person who had been selected” for tracking by the NSA, she told the Web site. The still-unidentified e-mailer told her at the time, “ ‘You probably don’t like how this system works, I think you can tell the story.’ ”

She said she was suspicious of the source’s information, fearing that it might be “entrapment.”

But after corresponding with Snowden, Poitras said she “had an instinct that it was legit.” She said she turned to “people who have experience” in reporting national security issues, including Gellman and Greenwald.

Gellman did not disclose this sequence of events, nor mention Poitras’s role, in a first-person story in The Post on Monday; he declined to say why in a brief interview.

Greenwald, contacted separately by Snowden in February, told the Huffington Post on Monday that he put off investigating the story, in part because the source wanted to communicate via an encryption program that Greenwald had trouble installing. When Poitras vouched for the source, Greenwald said, he began pursuing the story in earnest.

Martin Baron, The Post’s executive editor, said Poitras was “intimately involved in our getting the story. She didn’t write the story, and she didn’t edit it, but she worked with Bart Gellman to make sure we had it. The documents to which we gained access revealed a major surveillance program, and the story was accurate. We had no reservations about giving her a byline as credit for her role.”

Poitras went further in her interview in Salon, characterizing the story as an act of conscience. “It’s not okay that we have secret courts that have secret interpretations of secret laws,” she said. “What kind of democracy is that? I felt like, this is a fight worth having. If there’s fallout, if there’s blowback, I would absolutely do it again, because I think this information should be public. Whatever part I had in helping to do that I think is a service.”