'Gag' order: FBI confirms special secrecy agreements for agents in Clinton email probe

The FBI has confirmed to a senior Republican senator that agents were sworn to secrecy -- and subject to lie detector tests -- in the Hillary Clinton email probe, an extensive measure one former agent said could have a "chilling effect."

A July 1 letter sent by a senior deputy to FBI Director James Comey to Senate Judiciary Committee Chairman Charles Grassley, R-Iowa, detailed the restrictions on agents. The letter, reviewed by Fox News, confirmed agents signed a "Case Briefing Acknowledgement" which says the disclosure of information is "strictly prohibited" without prior approval, and those who sign are subject to lie detector tests.

"The purpose of this form is to maintain an official record of persons knowledgeable of a highly sensitive Federal Bureau of Investigation counterintelligence investigation," the agreement attached to the Grassley letter reads, "....I (FBI agent) also understand that, due to the nature and sensitivity of this investigation, compliance with these restrictions may be subject to verification by polygraph examination."

The measures show the extent to which the bureau has gone to keep additional details of the politically sensitive case from going public. While Comey has provided some information ‎on why the FBI did not opt to pursue charges, Attorney General Loretta Lynch repeatedly ducked questions on specifics of the case at a House hearing Tuesday.

A recently retired FBI agent, who declined to speak on the record, citing the sensitivity of the matter, said a "Case Briefing Acknowledgement" is reserved for "the most sensitive of sensitive cases," and can have a "chilling effect" on agents, who understand "it comes from the very top and that there has to be a tight lid on the case."

The former agent said the agreements can also contribute to "group think" because investigators cannot bounce ideas off other agents, only those within a small circle.

More