Glenn Greenwald has a mysterious 'second source' - and maybe more

Edward Snowden isn't the only person with access to government surveillance secrets who's been spilling them to journalists. Glenn Greenwald has an unidentified second source, and he told Brian Stelter about the effort to protect that person.

The mysterious second source is discussed toward the end of a new documentary, "Citizenfour," about Snowden and Greenwald.

How did Snowden react to the news that another person had come forward with information about the government's mass surveillance practices?

"He reacted with a huge amount of shock," Greenwald told Stelter.

"And I think he was also very gratified, because, as you point out, one of the principal motives that he had in coming forward so boldly, identifying himself publicly, instead of hiding, was to motivate other people to shine light on what the government is doing in the dark that never should have been kept secret in the first place."

Might Greenwald and his colleagues at The Intercept have more than two sources at this point?

"All I can say about that," he said, "is that one of the goals that we had very much in the forefront of our mind when we created The Intercept was to enable sources to come forward in a secure and safe way, so that they can do what they have always done, which is bring to the attention of journalists information that is in the public interest."