Since at least 2011, the FBI has adopted secret rules to spy on journalists who publish classified information and hunt down their anonymous sources.

The federal agency's covert tactics were revealed in classified FBI documents published Thursday by The Intercept, the online site co-founded by Glenn Greenwald who won a Pulitzer Prize for his role in publicizing the classified documents leaked by NSA whistleblower Edward Snowden.

The "Domestic Investigations and Operations Guidelines" includes an update on the use of National Security Letters, a demand to secretly obtain a person's private information. In this case: phone records.

The document spells out whose approval is needed when the FBI is seeking phone records of journalists or news organizations "to identify confidential news media sources."

Normally, the FBI must get the nation's top prosecutor, the Attorney General, to approve spying on journalists.

But the FBI can avoid that process by having its own general counsel approve it.

Trevor Timm, executive director of the Freedom of the Press Foundation, said he was surprised that these rules "explicitly give the FBI the go-ahead" to use this tactic against journalists -- because it was once reserved for hunting down terrorists and spies.

The FBI declined to comment on the document, but did provide a much-redacted version of the document that included the line, "If the NSL is seeking telephone toll records of an individual who is a member of the news media or a news organization," with the rest of the sentence blacked out.

The agency noted, however, that it can't investigate journalists just because news is leaked. A law must be broken -- such as leaking classified information.

News reports that expose information about the president, Congress and the federal government often cite anonymous inside sources. This type of leak often angers government officials, and triggers investigations into the leaks.

This FBI tactic worries people who have exposed government wrongdoing in the past.

"This is an assault on First Amendment," Thomas Drake told CNNMoney.

Drake is a former senior executive of the NSA who became a whistleblower to expose illegal activities at the spy agency. He was indicted under the Espionage Act, but most of the charges were later dropped and he pleaded guilty to unauthorized use of a computer.

"The intent here is to expose sources. It's the conveniently lazy way to conduct extra-legal access to data and surveillance. National Security Letters preempt the Constitution, the Fourth Amendment, and bypass judicial constraints and due process."

Not the first time

The federal government has been caught spying on journalists in the past. The Obama administration has used the Espionage Act to hunt down whistleblowers who leaked to journalists more than all previous presidential administrations combined, according to several studies by news organizations.

Federal agents spied on Fox News reporter James Rosen, tracking his movements, emails and phone records after he published classified information about North Korea in 2010. Agents also spied on two months of phone records from The Associated Press in 2013.

In the aftermath of these two embarrassing cases, the Obama administration touted new, more restricted guidelines in chasing the leak of classified information to journalists. Attorney General Eric Holder held a well-publicized meeting with editors in 2013 to say the Justice Department would change it ways.

But these newly revealed FBI documents show that agents can simply use an NSL to obtain phone records -- and circumvent the Obama administration's new guidelines.

Timm criticized how easy it is for the FBI to use NSLs.

"They're barely rules at all," Timm said. "All the FBI needs to use an National Security Letter against a journalist is extra sign-off from a couple of superiors."

There's also an irony in all this.

Timm tried to obtain a copy of these rules from the FBI using an open records request. He even sued the FBI last year to get them. (CNN filed an amicus brief supporting Timm in that lawsuit.)

But it took a leaker to show how the government is hunting down leakers.