didn't say they had to comply with congressional

It also said they could be polygraphed; it

The FBI had agents assigned to Hillary Clinton's email case sign a special secrecy agreement that bars them from talking about the investigation.

The law enforcement agency confirmed in a letter to top-ranking senators on the upper chamber's judiciary committee that agents signed a 'Case Briefing Acknowledgement' that reminded them of their obligation to protect classified and sensitive information and said they could be polygraphed.

In the letter the FBI said the document, which a senator on the receiving end said amounts to a 'gag' order, was not a 'unique circumstance; depending on the sensitivities in a given investigation, FBI employees may from time to time be asked to sign similar forms.'

However, former and current FBI agents said that the action is not a standard operating procedure.

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The FBI had agents assigned to Hillary Clinton's email case sign a special secrecy agreement that bars them from talking about the investigation - and warned they could be polygraphed

'This is very, very unusual. I’ve never signed one, never circulated one to others,' a former FBI chief told the New York Post.

A current agent told the news publication, 'I have never heard of such a form. Sounds strange.'

Another retired agent told Fox News that the agreement is only used in 'the most sensitive of sensitive cases.'

The document can have a 'chilling effect' on agents, the source said, as it means 'it comes from the very top and that there has to be a tight lid on the case.'

Former Arkansas Governor Mike Huckabee told Fox News' Megyn Kelly Wednesday night that the document's name was 'euphemistic' - 'it is a gag order,' the ex-presidential candidate said.

'That's like saying that Thelma and Louise actually just had a spontaneous steering malfunction. It's absurd.'

Huckabee told her the non-disclosure agreement is unusual because FBI agents are 'already prohibited from talking out of school....any FBI agent who has taken the oath to be an FBI agent is already covered by this.'

' "This was an in your face,' according to FBI agents with whom I spoke,' Huckabee said.

The FBI's July 1 letter to Senate Judiciary Chairman Chuck Grassley and Ranking Member Patrick Leahy is the first documented admission by the bureau that agents had to sign the special non-disclosure agreement.

'These forms served two purposes: to maintain an official record of all persons knowledgeable of this highly sensitive investigation, and to remind individuals of their obligations to protect classified and sensitive information,' the letter, sent by one of the FBI's congressional liaisons, said.

It went on to say, 'No one refused to sign the acknowledgement or raised any questions or concerns about doing so.'

Agents were required to sign two forms. The second was an addendum to the first that came about after Grassley sent a letter on Feb. 4 asking about whistleblower protections and congressional testimony.

The original form - published online by Fox News - contained the records keeping clause and said: '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 Republican senator who received the confirmation from the FBI that agents had to sign the agreement wants the bureau's director, James Comey, seen here testifying before Congress last week, to release all the materials related to the case

The addendum states that the agreement does not supersede or interfere with any 'existing statute or Executive order relating to (1) classified information, (2) communications to Congress, (3) the reporting to an Inspector General of a violation of any law, rule, or regulation, or mismanagement, a gross waste of funds, an abuse of authority, or a substantial and specific danger to public health or safety, or ( 4) any other whistleblower protection.'

Grassley subsequently lit up FBI Director James Comey in a letter that said it is 'troubling' that the FBI 'tried to gag its agents with a non-disclosure agreement on this matter, in violation of whistleblower protection statutes.'

The Republican senator pointed out that the original document 'failed to exempt protected whistleblowing' and was revised only after wrote to him 'to you did you advise your FBI agents that they are still free to speak with Congress regarding waste, fraud, and abuse.'

He also noted that it took the FBI five months to send a partial reply to his letter on July 1 and a full response on July 5.

The FBI 'should release in detail the actual evidence it gathered in the course of the investigation,' he demanded, including thousands of new emails from Clinton it found that were not among those that she provided to the State Department.

'Congress and the people have a right to know the full set of evidence on which you based your decision,' Grassley told Comey in the letter, also published by Fox. 'Until the FBI does so, much of the public will rightly be skeptical of the integrity of this investigation.'

Yesterday the State Department said it would make those emails public like it did with the ones she did turn over to the government.

It did not, however, provide a timeline for doing so, meaning the emails could stay under wraps until after November's presidential election.

Comey recommended last week that Clinton not be charged with a crime for mishandling classified information. Justice Department head Lorretta Lynch said she would not prosecute the former secretary of state based on the outcome of the probe.