Hillary Clinton has urged the FBI to explain without delay why it is carrying out a new investigation into her use of emails.
Americans “deserve to get the full and complete facts immediately,” she said.
The FBI says it has fresh emails which may be “pertinent” to a previous inquiry into her use of a private server when she was secretary of state.
Republican presidential rival Donald Trump said the move was the “biggest political scandal since Watergate”.
The FBI’s announcement was made on Friday, just 11 days before the election.
The latest emails were discovered as part of a separate investigation into the estranged husband of top Clinton aide, Huma Abedin.
Devices belonging to her and Anthony Weiner, a former high-flying congressman, 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 discovery of the new emails was revealed by FBI director James Comey in a letter to Congress on Friday.
Mr Comey said while he did not yet know the significance of the findings, the FBI would investigate if any of the emails contained classified information.
Democrat presidential candidate Mrs Clinton said she was “confident” the investigation into the emails would not change the FBI’s original finding in July.
The FBI decided she should not be prosecuted for her use of a private email server as secretary of state – a position she held between 2009 and 2013.
“It’s imperative that the bureau explain this issue in question, whatever it is, without any delay,” she told reporters on the campaign trail in Des Moines, Iowa, on Friday.
A senior media adviser on her campaign, James Rubin, told the BBC on Saturday that “when this is all over, this is going to be a gigantic tempest in a teapot”.
He also suggested the FBI decided to send the letter now to prevent it being criticised by the Republicans after the election.
For his part, Donald Trump told a crowd of supporters at a rally in Iowa: “It’s everybody’s hope that justice at last can be delivered.”
“The FBI would never have reopened this case at this time unless it were a most egregious criminal offence.”
He compared the latest development to that of the 1970s Watergate scandal that engulfed then-Republican President Richard Nixon.
‘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 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.
Clinton campaign chairman John Podesta criticised the FBI’s “extraordinary” timing.
Opinion polls suggest Mrs Clinton is four points ahead of Mr Trump, according to a Real Clear Politics average.
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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 “email@example.com” 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.
FBI Director James Comey’s letter to Congress
Predict the president
US election 2016: Clinton demands details of new email probe