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.
"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".
'October surprise' for Clinton - US media react
- "The question will be how badly damaged was Ms Clinton's candidacy by the 11th-hour re-eruption of a controversy that never should have generated so much suspicion or accusation in the first place" - Wall Street Journal
- "Mrs Clinton's apparent effort to blunt scrutiny by means of that private server has only led to far more damaging scrutiny and suspicion, with no end in sight" - New York Times
- "It would be nice if Comey could expedite this "overtime" investigation: Having it run past 8 November raises even more complications" - New York Post
- "Dashed are the hopes that the campaign could come to a conclusion on a high note, instilling in Americans a feeling that casting a history-making vote for Clinton is something more than merely a repudiation of Donald Trump" - Politico's Annie Karnie
- "If past is prologue, and it usually is, then a Hillary Clinton presidency may be engulfed and disabled by scandal. As a consequence, she is likely to accomplish little on behalf of the American people. In other words, her presidency could be dead on arrival" - Fox News' Gregg Jarrett
- "Hillary Clinton's biggest concern will not be the very remote possibility of future criminal charges, but the much likelier chance that a few too many voters in key states will have second thoughts about pulling the lever for a presidential contender still under suspicion of wrongdoing by the FBI" - CNN's Buck Sexton
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.
Paul Ryan, the highest-ranked elected Republican, called the FBI decision "long overdue".
The House of Representatives Speaker renewed his call for the Director of National Intelligence to halt classified briefings for the Democratic candidate.
"She was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information," he said.
The former secretary of state's private email server was first revealed in March 2015 by the New York Times.
She did not immediately express regret, and said the main reason for her "firstname.lastname@example.org" address was "convenience".
Soon after that she apologised in an interview with ABC News, and has since said sorry to voters a number of times.