Reporters have long known it to be true. When they asked a federal employee a question, they could often hear the fear in the employees' voices as the employees said they could not talk without press office permission. Now there is research proving what we have known all along, thanks to a survey from the Society of Professional Journalists (SPJ).
Most federal agency press offices shadow their employees to keep them from talking to reporters almost as diligently as the "minders" that once followed nuclear scientists in Saddam Hussein's Iraq. The Bush administration asserted that the minders policy was evidence that Saddam had something to hide. 
Over the years, the Society of Environmental Journalists has taken strong stands against this kind of interference  with journalists' ability to do their jobs of informing the public, especially at EPA.
SPJ commissioned work by survey research professionals canvassing 776 newsgatherers (of whom 146 completed the survey) during January-February 2012. Here are some of the findings:
- "Pre-approval: Three-quarters reported that they have to get approval from public affairs officers before interviewing an agency employee (a third said that occurs all of the time and 45 percent some of the time)."
- "Prohibition: About half the reporters said agencies outright prohibit reporters from interviewing agency employees altogether at least some of the time, and 18 percent said it happens most of the time."
- "Routing: Seven out of 10 reporters say their requests for interviews are forwarded to public affairs officers for selective routing to whomever they want."
- "Monitoring: About 16 percent of the reporters said their interviews are monitored in person or over the telephone all the time, a third said it happens most of the time and another third said it happens some of the time."
See the full report: "Mediated Access: Journalists’ Perceptions of Federal Public Information Officer Media Control,"  Society of Professional Journalists, March 12, 2012, by Carolyn Carlson, David Cuillier, and Lindsey Tulkoff.