Hillary Clinton's infamous private email account housed classified information so sensitive that the words 'top secret' don't do them justice.

That assessment came last Thursday in a letter to members of Congress from the U.S. Intelligence Community's inspector general, who described sworn affidavits from an intelligence professional involved with evaluating Clinton's emails at the State Department.

'Several dozen' of the messages, part of a cache which Clinton handed over more than a year ago after she deleted tens of thousands of messages she considered 'personal' in nature, merit the label 'classified,' Charles McCullough wrote in the letter.

And some, he said, should be classified at the 'top secret / SAP' level. 'SAP' stands for 'Special Access Programs,' a tightly controlled category of top-secret material that's usually shown to government officials on a 'need-to-know' basis.

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TROUBLE: Hillary Clinton's reputation took another hit Tuesday, and the potential criminal case against her grew, as a letter from the Intelligence Community's inspector general spelled out the higher-than-top-secret nature of some of the secrets in her personal emails

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Giving material that designation, the Fox News Channel reported Tuesday, indicate that compromising it could endanger an entire stream of intelligence collection – or even the safety of the people involved.

SAP intelligence 'always has specialized controls on it,' a retired intelligence official told DailyMail.com on Tuesday, 'meaning that even if you're the DNI [Director of National Intelligence], you can't just pull it up on your computer.'

'You would have to look at it in a SCIF, or maybe on a secure aircraft if you were Hillary Clinton. But the idea that SAP intel could ever wind up in an email is horrifying.'

The retired official, a 30-year veteran, requested anonymity so he could speak freely.

A SCIF, shorthand for a Sensitive Compartmented Information Facility, is a hardened room inside a government building, aircraft or ship that is restricted to people with 'SCI' clearance and protected against electronic intrusion.

People entering SCIFs, the intelligence official said, are asked to surrender cell phones, laptops, tablets and any other device 'that could copy or transmit whatever you're looking at.'

In addition to 'SCI' materials, he explained, SAP designations can apply to a wide range of intelligence including nuclear and biological weapons data and specialized programs run directly out of the West Wing of the White House.

Only the president; the secretaries of state, defense, energy and homeland security; the attorney general; and the director of national intelligence – and their principal deputies – can designate intelligence as part of an SAP.

In December 2009, President Barack Obama signed an executive order outlining the justification for creating an SAP, saying it was reserved for cases where 'the vulnerability of, or threat to, specific information is exceptional,' and 'the number of persons who ordinarily will have access will be reasonably small and commensurate with the objective of providing enhanced protection for the information involved.'

McCullough's letter, sent January 14 to the chairmen of the Senate intelligence and foreign relations committees, says he has 'received two sworn declarations from one [intelligence community] element' concerning the Clinton emails.

An 'element' can be any of 17 federal agencies, including the CIA, the NSA and a host of other law enforcement and military offices.

'These declarations cover several dozen emails containing classified information determined by the IC element to be at the confidential, secret, and top secret/sap levels,' the letter read.

'According to the declarant, these documents contain information derived from classified IC element sources.'

So far, IC agencies have identified 1,340 emails among Clinton's personal collection that they retroactively designated 'classified.'

Clinton has insisted that nothing she sent or received was 'marked classified' at the time she interacted with it on a home-brew email server she used exclusively while she was secretary of state.

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The Republican National Committee made immediate hay over the Fox News report.

'Hillary Clinton's decision to skirt government transparency laws with her secret email server has taken on an entirely new level of recklessness,' RNC spokesman Michael Short said Tuesday.

'The revelation that Clinton exposed intelligence from our most secretive and highly classified programs raises serious legal questions given the fact she signed non-disclosure agreements obligating her to protect classified information regardless of whether it was marked.'

Fox interviewed a former senior law enforcement official with what the network called 'decades of experience investigating violations of SAP procedures.'

'There is absolutely no way that one could not recognize SAP material,' that source said. 'It is the most sensitive of the sensitive.'

Clinton is campaigning for president while under a cloud of suspicion and an ongoing FBI investigation.