Hillary Clinton says she is “confident” a new FBI probe linked to her emails will not change its original finding that she should not be prosecuted.
The Democratic presidential candidate called on the FBI director to explain the new inquiry to the American people.
James Comey earlier said the FBI was looking into newly found messages.
The latest emails came to light during a separate inquiry into top Clinton aide Huma Abedin’s estranged husband, former congressman Anthony Weiner.
Devices belonging to Ms Abedin and Mr Weiner were seized in an investigation into whether he sent sexually explicit emails to a 15-year-old girl in North Carolina.
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“The American people deserve to get the full and complete facts immediately,” said Mrs Clinton.
“It’s imperative that the bureau explain this issue in question, whatever it is, without any delay.”
She highlighted that Mr Comey had said he did not know the significance of the new emails, adding: “I’m confident (that) whatever they are will not change the conclusion reached in July.”
Mrs Clinton’s Republican rival Donald Trump, however, described the FBI investigation as “the biggest political scandal since Watergate”, referring to the 1970s scandal that engulfed Republican President Richard Nixon.
“It’s everybody’s hope that justice at last can be delivered,” he told supporters at a rally in Iowa.
“The FBI would never have reopened this case at this time unless it were a most egregious criminal offence.”
Mr Comey said the FBI would investigate if the newly discovered emails contain classified information.
The FBI chief said in a letter to Congress that investigators had discovered the emails “in connection with an unrelated case… that appear to be pertinent to the investigation”.
He said he “cannot yet assess whether or not this material may be significant, and I cannot predict how long it will take us to complete this additional work”.
How big is this? Anthony Zurcher, BBC News, Washington
It could be nothing. It could be everything. And it almost certainly won’t be resolved before Americans head to the polls in just under two weeks.
The letter from Mr Comey to Congress is frustratingly vague. There’s no information, for instance, or how many emails are in question. That will only fuel the rampant speculation already breaking out, with leaks from “government sources” in the coming days sure to fan the flames.
Mrs Clinton’s critics will go on the attack, using the latest news to support their claims that the former secretary of state engaged in malfeasance. Her supporters will spend the next few days in a defensive crouch, trying to assess how bad the damage could be.
What’s certain, however, is that whether this turns out to be a big deal or not, it places the spotlight on all the wrong places for the Clinton campaign. It all but guarantees that even if she wins White House, the early days of her presidency will be dogged by this long-running political imbroglio.
The FBI has already established the Democratic candidate had classified information on a private email server.
In July, Mr Comey said Mrs Clinton’s handling of sensitive material during her 2009-13 tenure as secretary of state was “extremely careless”, but cleared her of any criminal wrongdoing.
The revelation that she handled sensitive information while breaking federal rules by running her own email server out of her upstate New York home has dogged her campaign since last year.
With Clinton team – Chris Gibson, BBC News
What a difference a plane journey makes. When we left Westchester, New York, on Hillary Clinton’s campaign plane, spirits were high amongst her staff.
Her campaign manager, Robby Mook, came to the back of the plane and told reporters that early voting was going in their favour and that Mrs Clinton would even campaign in Republican-leaning Arizona next week.
But Mr Mook added that they weren’t taking anything for granted. He said: “Hillary is superstitious.”
When we landed and wifi internet was restored, the Clinton team first learned the news of a reopened FBI investigation.
She was right to be superstitious.
As we got off the plane, one of her advisers told us: “We are just learning about this at the same time as you are.”
Mrs Clinton stayed on for far longer than usual. Her team were digesting the news and working out how to respond.
When she did walk off the plane, she smiled and ignored shouted questions from the media on the tarmac about the long-running email saga.
Clinton campaign chairman John Podesta criticised the FBI’s “extraordinary” timing.
The revelation comes just 11 days before Americans go to the polls in the presidential election.
Mrs Clinton is five points ahead of Mr Trump, according to a Real Clear Politics average.