A second unit within the State Department -- led by its top internal auditor -- conducted a separate probe into the mysterious deletion of a critical exchange about Iran from the department's video of its December 2, 2013 daily press briefing, Fox News has learned.

In an email to Fox News Friday afternoon, Doug Welty, a spokesman for the State Department's Office of the Inspector General (OIG), confirmed that investigators there launched their own "preliminary assessment" of the case, including conducting their own interviews with State Department employees and their own review of emails and other documents. 

Last week, the State Department's Office of the Legal Advisor (OLA), the department's top attorney, released a report that found -- after a three-month investigation that included email and document reviews and interviews with nearly three dozen DOS employees -- that it was impossible to determine whether the deletion was caused by a technical glitch or by willful misconduct.

Welty's statement marked the first public confirmation by any officials at the State Department that OIG had been reviewing the case separately from OLA. 

House Oversight Chairman Jason Chaffetz, R-Utah, told Fox News he and his colleagues were "very frustrated" by the investigative efforts of both OLA and OIG, and that they plan, as part of House Republicans' own investigation into the matter, to press both offices for access to their raw investigative files. The first request the lawmakers make will be for voluntary production by the offices.

In presenting the OLA's findings to reporters last Thursday, State Department spokesman John Kirby said there was "no evidence" as to why the edit was made -- despite the fact that the OLA report itself acknowledged, in a section titled "Evidence of Purposeful Editing," that the individual who ordered the edit specifically mentioned "a Fox network reporter and Iran" while issuing the order to the subordinate technician who carried it out.

"The results of our preliminary assessment show that limited evidence exists surrounding the December 2 DPB (daily press briefing) and that the available facts are inconclusive," said Welty of OIG. "However, the identification of the missing footage prompted the Department to improve its video policies. Specifically, the Department explicitly prohibited DPB content edits and is currently working with NARA to schedule the DPBs for disposition as federal records."

OIG indicated that due to the paucity of evidence -- the State Department has said it can no longer access the relevant telephone records for the technician -- additional investigation would likely be fruitless. "No further work by OIG would add clarity to the events surrounding the missing footage," Welty said, "or effect any additional change at the Department."

Officials at OIG privately briefed key congressional Republicans on their work last week, making it clear the internal auditors would not carry the investigation any further.

At the briefing in question, then-State Department spokeswoman Jen Psaki was asked by Fox News’ James Rosen about an earlier claim that no direct, secret talks were underway between the U.S. and Iran – when, in fact, they were.

Psaki at the time seemed to admit the discrepancy, saying: “There are times where diplomacy needs privacy in order to progress. This is a good example of that.”

However, Fox News later discovered the Psaki exchange was missing from the department’s official website and its YouTube channel. Eight minutes from the briefing, including the comments on the Iran deal, were edited out and replaced with a white-flash effect.

Fox News’ James Rosen contributed to this report