At least one of the emails on Hillary Clinton's private server contained extremely sensitive information identified by an intelligence agency as "HCS-O," which is the code used for reporting on human intelligence sources in ongoing operations, according to two sources not authorized to speak on the record.

Both sources are familiar with the intelligence community inspector general’s January 14 letter to Congress, advising the Oversight committees that intelligence beyond Top Secret -- known as Special Access Program (SAP) -- was identified in the Clinton emails, as well the supporting documents from the affected agencies that owned the information and have final say on classification.

According to a December 2013 policy document released by the Office of the Director of National Intelligence: This designation "is used to protect exceptionally fragile and unique IC (intelligence community) clandestine HUMINT operations and methods that are not intended for dissemination outside of the originating agency.”

It is not publicly known whether the information contained in the Clinton emails also revealed who the human source was, their nationality or affiliation.

Dan Maguire, former Special Operations strategic planner for Africom, told Fox News the disclosure of sensitive material impacts national security and exposes U.S. sources.

"There are people’s lives at stake. Certainly in an intel SAP, if you’re talking about sources and methods, there may be one person in the world that would have access to the type of information contained in that SAP,” he said.

It is not known what the impact was on the source, nor the findings of a damage assessment by the agency that controlled the source.

Separately, Fox News has learned that the so-called "spillage" of classified information is greater than the “several dozen” emails identified in the January 14 letter to Congress, which also acknowledged for the first time, that the Clinton emails contained intelligence beyond Top Secret, also known as Special Access Programs (SAPs).

The source said that the "several dozen" refers to the main or principal email thread identified by reviewers, not the number of times that classified information was forwarded, replied to or copied to people who did not have a “need-to-know” using unsecured communication channels -- in this case a personal server. More than one Special Access Program was affected.

"It's pretty tough to have SAP program material out in the public domain. I mean, it's a huge foul if that occurs,” said Maguire, who retired after 46 years of service, and who was involved with Special Access Programs throughout his career. Maguire says a damage assessment to the program is mandatory and immediate.

"It's a fairly laborious investigation. Once you know something was out to one person, that person sends it to 15, 15 send it to someone else -- so it's very difficult to ascertain where it all went but that's all part of the damage control aspect to get all the information back in the box."

The two declarations provided to the heads of the House and Senate Intelligence committees -- as well as the leadership of Senate Foreign Affairs with oversight for the State Department -- include the emails containing SAP intelligence, as well as supporting documents from the agency affected, showing how they reached the determination it came from one of its sources, and not from publicly available information.

When the inspector general’s letter was first reported by Fox News, Hillary Clinton spokesman Brian Fallon said, “This is the same interagency dispute that has been playing out for months, and it does not change the fact that these emails were not classified at the time they were sent or received.”