Up to 15 emails may be missing from the documents Hillary Clinton handed over to the State Department. Getty

Up to 15 work-related emails from Hillary Clinton’s tenure as secretary of state were missing from the archive Clinton handed over to the State Department earlier this year. The emails in question -- which are from before the Sept. 11, 2012, attacks in Benghazi, Libya, that killed U.S. Ambassador Christopher Stevens and three others -- are reportedly between Clinton aides and longtime Clinton political confidant Sidney Blumenthal.

In the aftermath of the attacks, Blumenthal, who reportedly received $10,000 a month from the Clinton family’s philanthropic organizations, allegedly helped Clinton try and tame the media backlash for the attacks in Libya. Blumenthal also sent Clinton intelligence memos on Libya, though he told the congressional panel investigating the attacks that he did not write the briefings.

Clinton campaign spokesperson Nick Merrill responded to the allegations of missing emails, saying “she has turned over 55,000 pages of materials to the State Department, including all emails in her possession from Mr. Blumenthal," according to a statement made to the Associated Press.

The former U.S. senator has been under fire for her use of a private email server during her time as Secretary of State during the first half of President Barack Obama’s presidency. The House committee investigating the Benghazi attack discovered the missing emails in March and issued a subpoena for the documents. In the aftermath of the subpoena, Clinton was widely criticized for keeping the emails in a private server, with some critics claiming that she had done so in order to dodge public information requests on her tenure as secretary.

In response, Clinton announced that she would release the emails to the State Department. However, she said she planned on destroying or keeping personal emails that were not relevant to the job before handing over 55,000 emails to the Department. On Thursday, State Department officials told the Select Benghazi Committee that they were no longer certain that the totality of the emails was available in the documents turned over by Clinton.

Blumenthal was barred from working in the Obama administration but was acting as an unofficial aide to Clinton through the emails. Clinton is currently running for the Democratic nomination for president.