It's not as simple as just blowing the whistle on the National Security Agency’s massive collection of data on phone calls anymore.

This explains better than I have seen before what  NSA fears he might have available to spill further, and even Greenwald does seem to agree that he might have some stuff that would be dangerous if released, but implies he is just holding it as a bargaining chip and never intended it to be released unless something happens to him:

Greenwald: Snowden’s Files Are Out There if ‘Anything Happens’ to Him
by Eli Lake, The Daily Beast, Jun 25, 2013 1:36 PM EDT

Snowden has shared encoded copies of all the documents he took so that they won’t disappear if he does, Glenn Greenwald tells Eli Lake.

Sorry, but can't get away from thinking of the word blackmail after reading the whole thing.

Especially if these parts are true:

A former U.S. counterintelligence officer following the Snowden saga closely said his contacts inside the U.S. intelligence community “think Snowden has been planning this for years and has stashed files all over the Internet.” This source added, “At this point there is very little anyone can do about this.”

“I don’t know for sure whether [Snowden] has more documents than the ones he has given me,” Greenwald said. “I believe he does. He was clear he did not want to give to journalists things he did not think should be published.”

Greenwald said he would not have published some of the stories that ran in the South China Morning Post. “Whether I would have disclosed the specific IP addresses in China and Hong Kong the NSA is hacking, I don’t think I would have,” Greenwald said. “What motivated that leak though was a need to ingratiate himself to the people of Hong Kong and China.”

For now, the FBI has taken a keen interest in the leak of FISA court documents. Those documents are some of the most closely guarded secrets in the U.S. intelligence community. As of last week, the FBI was investigating whether Snowden may have obtained those documents from a leak inside the secret FISA court.

Last week NSA Director Keith Alexander told the House Permanent Select Committee on Intelligence that Snowden was able to access files inside the NSA by fabricating digital keys that gave him access to areas he was not allowed to visit as a low-level contractor and systems administrator.