"To the extent that you have aided and abetted Snowden, even in his current movements, why shouldn't you, Mr. Greenwald, be charged with a crime," asked David Gregory on Meet the Press this weekend.

Glenn Greenwald (Associated Press.)

Glenn Greenwald, the journalist who over the last three weeks has broken a series of major stories about National Security Agency spying, called it "pretty extraordinary that anybody who would call themselves a journalist would publicly muse about whether or not other journalists should be charged with felonies."

Put aside the "should." The more chilling question, in certain ways, is the "could." The law is surprisingly murky on whether someone in Greenwald's position could be prosecuted. It's possible that, under certain circumstances, journalists could face jail time for doing investigative journalism.

The First Amendment protects freedom of the press, and you might think that would immunize reporters who publish classified information. Experts agree that the First Amendment provides journalists with robust protection if they merely publish information they passively receive from confidential sources.

But "passive" is not generally how investigative journalism works. Good reporters seek out information from their sources, promising them confidentiality in return. Experts say that if a journalist actively solicits classified information from a government employee, he might be on shakier legal ground.

Stephen Vladeck, a legal scholar at American University, says that sources themselves have little protection under the First Amendment. A 2006 Supreme Court ruling suggested that government employees (and contractors) like Edward Snowden effectively give up their First Amendment rights when they take a government job. That principle has made possible the Obama administration's aggressive prosecution of leakers.

Journalists and other private citizens do enjoy full First Amendment rights. In many circumstances, that's sufficient to immunize them for publishing classified information. But there are two important exceptions.

The first is if a publication poses a "clear and present danger of grave harm to the nation." Publication of troop movements or the identities of covert operatives, for example, could subject a journalist to criminal prosecution.

The other possible exception, which University of Chicago legal scholar Geoffrey Stone calls an "interesting unresolved puzzle," is when a journalist actively solicits a leak from a source.

"If the reporter bribes somebody to leak information, that's a crime," Stone said. "The question is at what point, if any, does solicitation cross the line?"

No journalist has ever been indicted for helping a source distribute classified information to the public. And Vladeck believes that's not a coincidence.

Cases against journalists "will never be brought," he said. "The government does not want to litigate whether private citizens are protected by the First Amendment."

But relying on executive branch discretion isn't a good strategy for protecting freedom of the press. Reporters need confidence that, regardless of who is president, they won't face felony prosecutions just for doing their jobs.