The U.S. Defense Department has found an email chain that Hillary Clinton failed to turn over to the State Department, officials said on Friday, despite her saying she had provided all work-related emails from her time as secretary of state. The disclosure adds to questions related to the Democratic presidential front-runner's unusual usage of a private email account and server while in government.

The messages were exchanged with Gen. David Petraeus when he led the military's U.S. Central Command, responsible for running the wars in Iraq and Afghanistan. They began before Clinton entered office and continued into her first days at the State Department. They largely pertained to personnel matters and don't appear to deal with highly classified material, State Department officials said, but their existence challenges Clinton's claim that she has handed over the entirety of her work emails from the account.

Republicans have raised questions about thousands of emails that she has deleted, as well as other messages that have surfaced independently of Clinton and the State Department. Speaking of her emails on CBS' "Face the Nation" this week, Clinton said, "We provided all of them." But the FBI and several congressional committees are investigating.

The State Department's record of Clinton emails begins on March 18, 2009 — almost two months after she entered office. Before then, Clinton has said she used an old AT&T Blackberry email account, the contents of which she no longer can access.

The Petraeus emails, first discovered by the Defense Department and then passed to the State Department's inspector general, challenge that claim. They start on Jan. 10, 2009, with Clinton using the older email account. But by Jan. 28 — a week after her swearing in — she switched to using a private email address on a home-based server that she relied on for the rest of her tenure. There are less than 10 emails back and forth in total, officials said, and the chain ends on Feb. 1.

The officials weren't authorized to speak on the matter and demanded anonymity. But State Department spokesman John Kirby confirmed that the agency received the emails in the "last several days" and that they "were not previously in the possession of the department."

Kirby said they would be subject to a Freedom of Information Act review like the rest of Clinton's emails. She gave the department some 30,000 emails last year that she sent or received while in office, and officials plan to finish releasing all of them by the end of January, after sensitive or classified information is censored. A quarter have been made public so far.

Additionally, Kirby said the agency will incorporate the newly discovered emails into a review of record retention practices that Clinton's successor, Secretary of State John Kerry, initiated in March. "We have also informed Congress of this matter," he added.

These steps are unlikely to satisfy Clinton's Republican critics.

Clinton has been dogged for months by questions about her email practices. She initially described her choice as a matter of convenience, but later said she had made a wrong decision.

Wire services