Hillary Clinton. Thomson Reuters Emails that passed through Hillary Clinton's private server while she served as US secretary of state reportedly contained intelligence so sensitive that it has since been marked beyond "top secret," Fox News reported on Tuesday.

The network reported on the contents of a letter authored by Intelligence Community Inspector General I. Charles McCullough III. The January 14 letter to senior lawmakers has not yet been made public.

"To date, I have received two sworn declarations from one [intelligence community] element. 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.

"SAP levels" refers to Special Access Programs (SAPs) that are meant to safeguard information deemed more classified than even "top secret."

"It is the most sensitive of the sensitive," a former senior law-enforcement officer told Fox News. "There is absolutely no way that one could not recognize SAP material."

Intelligence is allocated to a special-access program when "the vulnerability of, or threat to, specific information is exceptional," according to Executive Order 13526, which was signed by US President Barack Obama in late 2009 and details how to properly handle and protect classified national-security information.

Clinton signed a nondisclosure contract when she got to the State Department, in which she agreed to "never divulge anything marked SCI [sensitive compartmented information] or that I know to be SCI to anyone who is not authorized to receive it without prior authorization from the US Government department or agency that authorized my access."

The FBI and the State Department had no immediate comment on the report. A Clinton campaign spokesman didn't immediately respond to a request for comment from Business Insider.

Former Secretary of State Hillary Clinton. REUTERS/Scott Morgan

The Federal Bureau of Investigation has been looking into whether classified material was mishandled during Clinton's tenure at the State Department from 2009 to 2013.

The letter sent by McCullough — who revealed shortly after the probe began in August that two top-secret emails had been found on Clinton's server — was sent days after reports emerged that the FBI was widening its probe into Clinton's emails to determine whether any public-corruption laws were violated during her time at the State Department.

The FBI has reportedly been investigating a possible overlap of the Clinton Foundation charity with State Department business, Fox reported earlier this month, citing three unidentified intelligence officials.

Facing criticism last year for exclusively using a private server during her time as secretary of state, Clinton handed over about 30,000 work-related emails for the State Department to make public. She deleted about 31,000 more emails she says were personal in nature.

After months of negative headlines that battered her campaign, the Democratic presidential front-runner apologized in September for her email arrangement. But she has insisted that she didn't violate protocol — noting that she used communication practices that were widespread across the federal government — or pass along material marked classified.

Still, reports that hackers in China, South Korea, Germany, and Russia tried to break into Clinton's server have raised questions about the kind of security precautions the 2016 presidential candidate and her team took to safeguard the sensitive information that sometimes passed through her inbox.

More than 1,200 emails in Clinton's server have been retroactively marked "classified" since the investigation began in August, according to Politico.