The State Department acknowledged Thursday it could not find any record of part or all of 15 emails from Hillary Clinton dating from her time as secretary of state, copies of which have been given to US lawmakers.
A stash of thousands of emails has been turned over to a congressional panel investigating the 2012 attack on a US mission in Libya by longtime Clinton confidante and friend Sidney Blumenthal.
But in comparing the Blumenthal cache with some 30,000 emails Clinton has given to the State Department, officials admitted there were "a limited number of instances, 15, in which we could not locate all or part of the content of a document from his production within the tens of thousands of emails she gave us."
"The substance of those 15 emails is not relevant to the 2012 attacks in Benghazi," the State Department official stressed, asking not to be named.
The email correspondence has triggered a political row after Clinton -- now bidding to be elected America's first woman president in 2016 elections -- revealed she had used a private server and email address during her time as secretary of state from 2009 to early 2013.
A first slew of 296 emails relating to Libya and the 2012 attack on the US mission in Benghazi in which four Americans were killed was made public in May after a judge ordered that they be released.
The next batch from some 55,000 pages of correspondence is due to be released by the State Department on June 30.
"We have confirmed that the emails Secretary Clinton provided the Department include almost all of the material" Blumenthal gave to the House select committee on Benghazi, a State Department official said.
And apart from the 15 that are missing, the State Department also has some correspondence that does "not appear" in what Blumenthal handed over to the committee.
- Lingering suspicions -
Nick Merrill, a spokesman for Clinton, said the former first lady "has turned over 55,000 pages of materials to the State Department, including all emails in her possession from Mr Blumenthal."
Another Clinton spokesman said the former top diplomat did not recognize many of the documents produced by Blumenthal and could not speak to their origin.
Clinton turned over paper copies of the 30,000 emails in 12 cardboard boxes, and says she destroyed all the rest as they were not related to diplomatic matters but were personal.
But the revelations have renewed long-held Republican suspicions that she and her husband, former president Bill Clinton, are unnecessarily secretive.
The Benghazi attack, in which ambassador Chris Stevens was killed, is likely to stalk her bid for the White House, amid Republican allegations that she aided a coverup of the true events.