WASHINGTON — The Obama administration has discovered a chain of emails that Hillary Clinton failed to turn over when she provided what she said was the full record of work-related correspondence as secretary of state. The disclosure adds to the 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. 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, but their existence challenges Clinton’s claim that she has handed over the entirety of her work emails from the account.