At least four classified Hillary Clinton emails had their markings changed to a category that shields the content from Congress and the public, Fox News has learned, in what State Department whistleblowers believed to be an effort to hide the true extent of classified information on the former secretary of state’s server.

The changes, which came to light after the first tranche of 296 Benghazi emails was released in May, was confirmed by two sources -- one congressional, the other intelligence. The four emails originally were marked classified after a review by career officials at the State Department. But after a second review by the department's legal office, the designation was switched to "B5" -- also known as "deliberative process," which refers to internal deliberations by the Executive Branch. Such discussions are exempt from public release.  

The B5 coding has the effect, according to a congressional source, of dropping the email content "down a deep black hole."

The four mails are separate and distinct from another group of emails identified by the Intelligence Community Inspector General as containing two messages with "Top Secret"  information.

A congressional source told Fox that a July 23rd letter to Congress from the Inspector General for the Intelligence Community made passing reference to the incident in the recommendations "...that State FOIA officials implement a dispute resolution process in regard to differences of opinion about classification levels and exemptions. State has not yet provided sufficient information for us to close this recommendations."

According to recent congressional testimony, at least one of the lawyers in the office where the changes were made is Catherine “Kate” Duval, who was at the IRS during the Lois Lerner e-mail scandal and now handles the release of documents to the Benghazi select committee. Duval once worked for the same firm as Clinton's private attorney David Kendall. 

Fox News is told there were internal department complaints that Duval, and a second lawyer also linked to Kendall, gave at the very least the appearance of a conflict of interest during the email review. A State Department spokesman did not dispute the basic facts of the incident, confirming to Fox News the disagreement over the four classified emails as well as the internal complaints. But the spokesman said the concerns were unfounded.

The whistleblowers told intelligence community officials that they did not agree with the B5 changes, and the changes had the effect of shielding the full extent of classified content on the server. The incident was referenced in a Washington Times report mid-August, but this is the first time fuller details have been available. Because the emails are now marked B5, or deliberative, it is impossible to know the content and relevance to the congressional and FBI investigations.

The internal State Department disagreement was so significant that it rose to the level of Under Secretary for Management Patrick Kennedy, who is deeply involved in the email controversy, as Clinton's server arrangement required his formal signoff or tacit approval. Asked who signed off on the private server on Tuesday, State Department spokesman Mark Toner said, "I personally don't know."

Conservative group Judicial Watch, which has more than a dozen civil suits in federal courts, is now seeking a deposition of Kennedy in a case scrutinizing Clinton aide Huma Abedin’s controversial status as a special government employee (SGE). “All these issues fall under his responsibility,” Judicial Watch investigator Chris Farrell said.

Asked to respond to the allegations, State Department spokesman John Kirby said, “the Department has complete confidence that its attorneys -- who are almost exclusively career Department lawyers -- perform to the highest professional and ethical standards, including in connection with the review and release of Secretary Clinton’s emails.” A State Department official added that the lawyers do not have the final say on the codes, emphasizing it is a “multi-step review.”

On the appearance of a conflict of interest, Kirby defended Duval as “an exceptional professional and has the Department's utmost confidence … No one at the Department should, in addition to this burden, have her integrity or her excellent work ethic impugned.”  And on the connection to Clinton attorney Kendall, “the mere fact of working at a firm does not itself constitute a conflict of interest.  This is a large firm, and we are not aware that any counsel working on Clinton-related matters at the Department did so prior to joining the Department.”

A search of this week’s 7,000-page release found 694 emails with the B5 coding, about 10 percent of the total.

Fox News’ Pamela Browne contributed to this report.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.