Mrs Clinton demands the FBI release details of its probe "without delay" as Donald Trump describes it as "bigger than Watergate".
Hillary Clinton has insisted she is "confident" new emails being investigated by the FBI will not change the result of previous probes.
In a letter made public on Friday, FBI director James Comey revealed the bureau would examine whether the emails contained sensitive government information.
Responding at a hastily arranged news conference, Mrs Clinton demanded the FBI disclose full details of its new probe, which came to light just 11 days before the election.
"The American people deserve to get the full and complete facts immediately," Mrs Clinton told reporters in Iowa.
"It's imperative that the bureau explains this issue in question, whatever it is, without any delay."
The FBI had concluded in July that Mrs Clinton was "extremely careless" in handling classified information while Secretary of State, with scores of emails on her private server containing highly classified information.
But the bureau did not recommend criminal charges.
In Mr Comey's latest letter to key Republicans, he admitted he did not yet know whether the new messages were significant.
Mrs Clinton added: "I'm confident (that) whatever they are will not change the conclusion reached in July."
The Democratic candidate also said she did not believe the fresh probe would affect the election.
"People a long time ago made up their minds about the emails," she said. "I think that's factored into what people think and now they're choosing a president."
The messages were discovered after the FBI seized four electronic devices belonging to Mrs Clinton's aide Huma Abedin and her husband Anthony Weiner, the New York Times reported
They were taken during an investigation into illicit text messages between the former Congressman and a 15-year-old girl.
An unnamed US official told the Associated Press news agency the new emails did not come from Mrs Clinton's private server.
The news prompted a gleeful response from Mrs Clinton's Republican rival Donald Trump, who told his supporters 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."
Earlier, Mrs Clinton's campaign chairman John Podesta said "it is extraordinary that we would see something like this just 11 days out from a presidential election".
Hillary Clinton has insisted she is "confident" new emails being investigated by the FBI will not change the result of previous probes.
In a letter made public on Friday, FBI director James Comey revealed the bureau would examine whether the emails contained sensitive government information.
Responding at a hastily arranged news conference, Mrs Clinton demanded the FBI disclose full details of its new probe, which came to light just 11 days before the election.
"The American people deserve to get the full and complete facts immediately," Mrs Clinton told reporters in Iowa.
"It's imperative that the bureau explains this issue in question, whatever it is, without any delay."
The FBI had concluded in July that Mrs Clinton was "extremely careless" in handling classified information while Secretary of State, with scores of emails on her private server containing highly classified information.
But the bureau did not recommend criminal charges.
In Mr Comey's latest letter to key Republicans, he admitted he did not yet know whether the new messages were significant.
Mrs Clinton added: "I'm confident (that) whatever they are will not change the conclusion reached in July."
The Democratic candidate also said she did not believe the fresh probe would affect the election.
"People a long time ago made up their minds about the emails," she said. "I think that's factored into what people think and now they're choosing a president."
The messages were discovered after the FBI seized four electronic devices belonging to Mrs Clinton's aide Huma Abedin and her husband Anthony Weiner, the New York Times reported
They were taken during an investigation into illicit text messages between the former Congressman and a 15-year-old girl.
An unnamed US official told the Associated Press news agency the new emails did not come from Mrs Clinton's private server.
The news prompted a gleeful response from Mrs Clinton's Republican rival Donald Trump, who told his supporters 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."
Earlier, Mrs Clinton's campaign chairman John Podesta said "it is extraordinary that we would see something like this just 11 days out from a presidential election".