Hillary Clinton's unsecured home server contained more than a dozen emails deemed "top secret"- one of the highest levels of classification in the US government, the White House has said.
State Department spokesman John Kirby said the documents were not marked classified at the time they were sent.
Mrs Clinton's use of a personal email as secretary of state has become an issue in her presidential campaign.
This is the first time her emails have been labelled classified at any level.
Messages were marked "top secret" because they would cause "exceptionally grave" damage to national security if disclosed, the State Department said.