Newly discovered emails to be checked for classified information
The FBI has opened a fresh investigation into Hillary Clinton's personal emails following the discovery of new messages.
FBI director James Comey said the probe will examine whether the new emails - connected to an "unrelated case" - contain sensitive government information.
However, in Mr Comey's letter to key Republican committee chairmen in the House of Representatives and Senate, he said he cannot yet tell whether the new messages are significant.
Mrs Clinton addressed supporters at a rally in Cedar Rapids, Iowa, earlier but made no mention of the investigation.
In a statement, her campaign chairman John Podesta said the FBI needs to immediately provide more information about the latest discovery.
He said: "Director Comey's letter refers to emails that have come to light in an unrelated case, but we have no idea what those emails are and the Director himself notes they may not even be significant.
"It is extraordinary that we would see something like this just 11 days out from a presidential election.
"We are confident this will not produce any conclusions different from the one the FBI reached in July."
In his letter, Mr Comey wrote: "In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation.
"I agreed 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."
The probe raises the possibility of the FBI reopening the criminal investigation into her email activities.
An unnamed US official told the Associated Press news agency the new emails did not come from Mrs Clinton's private server.
The New York Times has reported the messages were discovered after the FBI seized electronic devices belonging to Mrs Clinton's aide Huma Abedin and her husband Anthony Weiner.
The FBI spent about a year looking into her use of an unauthorised private email server for her work as US secretary of state between 2009 and 2013, after it emerged there were classified government secrets in some of her emails.
The announcement prompted an almost immediate response from Mrs Clinton's Republican rival Donald Trump, who claimed the development is "bigger than Watergate".
At a rally in New Hampshire, Mr Trump said: "I have great respect for the fact that the FBI and the Department of Justice are now willing to have the courage to right the horrible mistake that they made.
"Hillary Clinton's corruption is on a scale we have never seen. We must not let her take her criminal scheme into the Oval office."
He also said the political system "might not be as rigged as I thought", having previously complained Washington had rigged the system against him.
Kellyanne Conway, a spokesman for Mr Trump, said on Twitter "a great day in our campaign just got even better".
Others have queried the timing of the announcement. Republican John Cornyn, who represents Texas in the Senate, tweeted: "Why is FBI doing this just 11 days before the election?"
The White House said it was not given notice on the FBI's announcement and it would not change President Obama's opinion and view of Mrs Clinton.