The U.S. State Department really doesn’t want to say what apparently went wrong during its diplomats’ dramatic overland escape from Tripoli, Libya, last year, claiming an entire internal report on the incident is classified.

In late July the State Department’s Office of Inspector General published a short, mysterious “Management Alert” that said an “issue” had been identified with the evacuation plan that required “immediate action.”

The alert cited an unclassified summary of an internal report but didn’t describe the issue except to say the embassy “did not adequately prepare for or execute all aspects of its July 2014 evacuation." The OIG made recommendations “intended to promote the development and execution of effective EAPs [Emergency Action Plans] worldwide and to prompt practicable actions to address deficiencies associated with the evacuation of Embassy Tripoli."

After the publication of the “Management Alert,” the State Department’s Bureau of Diplomatic Security declined to elaborate on the “deficiencies” discovered. The OIG declined to comment to ABC News at the time, and neither current U.S. Ambassador to Tripoli Deborah Jones nor Col. Kenneth Detreux, the U.S. Marine who led crisis response during the Tripoli escape, were made available for comment.

In early September ABC News requested the OIG conduct a mandatory declassification review [MDR] of the full report on the 2014 incident – which can sometimes produce a version of a report in question, even if heavily redacted. This week the OIG responded: No dice.

“We have determined that the material in the report meets the classification requirements… and is therefore exempt from disclosure,” a letter from the OIG says. “It contains sensitive information on foreign relations or foreign activities of the United States, including confidential sources and contains sensitive information concerning vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans or protective services relating to national security. Therefore, your MDR request is denied.”

Armored State Department SUVs from Tripoli, Libya photographed by the State Department Inspector General in Tunis, Tunisia.(State Department Office of Inspector General) Armored State Department SUVs from Tripoli, Libya photographed by the State Department Inspector General in Tunis, Tunisia.

The State Department’s security operations in Libya have been under a microscope since the deadly 2012 attack on an American diplomatic facility in Benghazi, which claimed the lives of four Americans including Ambassador Chris Stevens. A State Department internal investigation of that incident found that there was a “systematic failure” by the department to address the security needs of the Benghazi facility.

A year and 10 months later, the July 2014 evacuation from Tripoli by American officials was prompted by “freewheeling militia violence” that presented a “very real risk to our personnel,” as Secretary of State John Kerry put it at the time. Militias were battling, in part, over Tripoli’s international airport not far from the U.S. Embassy.

The convoy of more than 150 people withdrew at dawn, attempting to keep a low profile in a caravan of armored vehicles, to make a 250-mile overland trip to Tunisia. The U.S. military provided airborne security for the convoy with two nearby F-16s, high-altitude surveillance platforms and two MV-22 Ospreys filled with additional Marines.

The white-knuckle evacuation was deemed a success afterwards, but the OIG report was the first to publicly raise questions about State’s actions – questions that remain unclear and unanswered.

lee.h.ferran@abc.com

Editor's Note: The original version of this report said ABC News requested the MDR in August. It was requested in early September.