FBI director James Comey has said the bureau is reopening its investigation into Hillary Clinton's emails.
Investigators have discovered new emails "in connection with an unrelated case... that appear to be pertinent to the investigation", he said.
Mr Comey said investigators would determine whether the emails contain classified information.
The FBI has already found the Democratic candidate had classified information on a private email server.
Mr Comey had previously called Mrs Clinton's handling of classified information "extremely careless", but cleared her of any criminal wrongdoing.
FBI Director James Comey said Friday the bureau is reviewing new emails related to Hillary Clinton's time as secretary of state, according to a letter sent to eight congressional committee chairmen, a surprise development with 11 days until the election.
After recommending this year that the Department of Justice not press charges against the Secretary of State, Comey said in the letter that "recent developments" urged him to take another look.
"In connection with an unrelated case, the FBI has learned of the existence of emails that appear pertinent to the investigation," Comey wrote the chairmen. "I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation."