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.”