An email chain involving Secretary of State Hillary Clinton and Richard Holbrooke, special envoy to Afghanistan, which was stored on Clinton's unsecured server, contained "Top Secret" information on a CIA asset in Afghanistan, according to an exclusive report by Fox News.

The discussion of a foreign national working with the U.S. government raises security implications – an executive order signed by President Obama said such unauthorized disclosures are “presumed to cause damage to the national security."

The U.S. government official said the Clinton email exchange, which referred to a New York Times report, was among 29 classified emails recently provided to congressional committees with specific clearances to review them. In that batch were 22 “top secret” exchanges deemed too damaging to national security to release.

Confirmation that one of these exchanges concerned a reported CIA asset means the emails went beyond issues like the drone strike campaign. Democrats repeatedly have said some messages referred to this, reinforcing Clinton's position that the documents are over-classified.

Based on the timing and other details, the email chain likely refers to either anOctober 2009 Times story that identified Afghan national Ahmed Wali Karzai, the half-brother of then-Afghan president Hamid Karzai, as a person who received “regular payments from the Central Intelligence Agency” -- or an August 2010 Times story that identified Karzai aide Mohammed Zia Salehi as being on the CIA payroll. Ahmed Wali Karzai was murdered during a 2011 shoot-out, a killing later claimed by the Taliban.

Fox News was told the email chain included then-Secretary of State Clinton and then-special envoy to Afghanistan and Pakistan Richard Holbrooke and possibly others. The basic details of this email exchange were backed up to Fox News by a separate U.S. government source who was not authorized to speak on the record.

It’s unclear who initiated the discussion – Clinton, Holbrooke or a subordinate – or whether the CIA's relationship with the Afghan national was confirmed, because the classified documents are not public.

Why wasn't the email clearly marked as being classified?  Is this another instance where a Clinton aide removed the heading so that it could be sent to an unsecure server?

The U.S. government official's account of the Clinton email chain dovetails with a Feb. 3 interview on Fox News’ “America's Newsroom,” where Republican Rep. Chris Stewart, a member of the House intelligence committee, said, "I have never read anything more sensitive than what these emails contain. They do reveal classified methods. They do reveal classified sources and they do reveal human assets."

Stewart added, "I can't imagine how anyone could be familiar with these emails, whether they're sending them or receiving them, and not realize that these are highly classified."

Although the law doesn't recognize the excuse that the emails weren't "marked" as classified, the severity of the charges could be based on what a reasonable, knowledgeable official would instantly recognize as classified information.  It shouldn't matter how the emails were headed.  A rookie State Department clerk would know immediately that mentioning a CIA asset would instantly classify the communication as at least "Top Secret."

As these revelations keep dribbling out, you can sense the noose tightening around Clinton.  But there is still no indication that Clinton herself is the target of an FBI investigation, although the use of her unsecure server is.  This leaves the question open whether the Obama Justice Department, despite overwhelming evidence of Clinton's guilt, would fail to throw the book at Clinton and charge her with mishandling classified information.