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".