Two secret letters the FBI sent to the State Department have revealed for the first time that the bureau's investigation into Hillary Clinton's private email server, and the classified emails sent through it, stemmed from a so-called "Section 811" referral from the Intelligence Community's Inspector General (ICIG). The ICIG determined that classified, national security information in Clinton's emails may have been "compromised" and shared with "a foreign power or an agent of a foreign power."

Section 811 of the Intelligence Authorization Act of 1995 "is the statutory authority that governs the coordination of counterespionage investigations between Executive Branch departments or agencies and the FBI." A Section 811 referral is a report to the FBI about any unauthorized information that may have been disclosed to a foreign power.

A Section 811 referral "arises whenever there is a compromise of classified information — for whatever reason," said Steven Aftergood, director of the Project on Government Secrecy at the Federation of American Scientists. "It could include espionage, but it could also include negligence, inadvertence, or something else.... Section 811 does not assert a violation of criminal law."

FBI Director James Comey, in an extraordinary public statement last month announcing that the FBI had closed its investigation, said it was possible that "hostile actors gained access" to Clinton's emails because she used her personal email "extensively" while traveling abroad. However, the FBI did not uncover "direct evidence" that Clinton's email server "was successfully hacked."

The two letters, dated October 23, 2015 and January 20, 2016, and marked "For Official Use Only," were written by Peter Strzok and Charles H. Kable IV, the section chiefs of the FBI's counterespionage section, and sent to Gregory B. Starr, the assistant secretary at the State Department's Bureau of Diplomatic Security. They were written while the FBI was investigating Clinton's use of an unsecure, private email server and the dissemination of classified information.

"The potential compromise was identified when, as part of a Freedom of Information Act (FOIA) request [by VICE News], the U.S. Department of State (DoS) and the ICIG reviewed electronic mail (email) communications from the private email accounts previously used by a former Secretary of State during her tenure at DoS," Kable wrote. "An initial review of this material identified emails containing national security information later determined by the US Intelligence Community to be classified up to the Top Secret/Sensitive Compartmented Information Level."

The letters were turned over to VICE News in response to a FOIA lawsuit we filed against the FBI last year seeking, among other records, correspondence between the FBI and the State Department about Clinton's private server and the FBI's probe into it. The FBI had previously said that if it were to disclose the contents of these letters — even the identities of the senders and receivers — it would jeopardize its investigation.

Kable had asked Starr to review a DVD the FBI sent containing the emails and to conduct a classification review of the communications. He requested that State "de-conflict" its response with any other classification review requests the State Department received from other government agencies.

"What [the de-confliction request] means is that FBI anticipates that different classification judgments may have been rendered concerning some of the individual [emails] under review," Aftergood said. "That's an extremely important point in this whole saga, in which there are conflicting official opinions about the classification status of various emails."

Last month, the FBI turned over to VICE News documents revealing that Clinton exchanged nearly two-dozen top secret emails from her private server in 2011 and 2012 with her deputy chief of staff, Jacob Sullivan, her chief of staff, Cheryl Mills, and Deputy Secretary of State William Burns. The State Department said the contents of the 22 emails were so highly classified that not even the subject matter could be disclosed.

Watch VICE News' Jason Leopold analyze the Vaughn index.

Separately, the FBI also released a previously secret declaration written by the bureau's FOIA chief, David Hardy, that was submitted earlier this year to a federal court judge presiding over VICE News' FOIA case. The declaration, which has been marked "Law Enforcement Sensitive," said the FBI did not correspond directly with Clinton or her representatives during its investigation.

The FBI spent a year probing Clinton's use of a private email server. Though Comey called Clinton and her aides "extremely careless in their handling of very sensitive, highly classified information," he recommended that no one be charged. The FBI officially closed its investigation July 5.

Following Comey's press conference, the State Department said it would reopen an internal review into any mishandling of classified information. Even though Clinton and her aides no longer work at the State Department, they could still be subject to a number of penalties, such as losing their security clearances.