The disclosure raises the possibility of the FBI reopening the criminal investigation involving the Democratic presidential nominee just days before the election, although it is not clear if that will happen.
Clinton's campaign didn't immediately respond to request for comment.
In a letter sent to congressional leaders, FBI Director James Comey says that new emails have come to light recently that have prompted investigators to take another look at the sensitive government information that flowed through the private email sever Clinton used while serving as secretary of state.
A U.S. official says the newly discovered emails came from a separate sexting probe of former congressman Anthony Weiner.
Federal authorities in New York and North Carolina are investigating online communications between Weiner and a 15-year-old girl.
The official said Friday that the emails referenced by Comey surfaced during that investigation. The official was familiar with the investigation but was not authorized to discuss the matter by name and spoke on condition of anonymity.
The New York Times first reported the connection.
