Director James Comey informed Congress in a letter that the FBI has learned of the existence of emails that appear to be pertinent to the investigation. Comey wrote that the discovery came in connection with an unrelated case.
Multiple sources told ABC News that the newly discovered emails came from a separate federal investigation of former Congressman Anthony Weiner. Federal prosecutors have been looking at whether Weiner sent an explicit text message to an underage girl in North Carolina. In the course of that investigation, ABC News was told emails were discovered on at least one device used by Weiner and his wife, Clinton aide Huma Abedin. She was among the handful of people that had an email address on Clinton's private server. The FBI is only just beginning the process of trying to look at these emails to determine whether they offer any new evidence in the Clinton matter.
In his letter, Comey wrote that "the FBI cannot yet assess whether or not this material may be significant."
Clinton campaign chair John Podesta issued an angry response to Comey's letter, calling on the FBI to release more details to the American public.
"Director Comey's letter refers to emails that have come to light in an unrelated case, but we have no idea what those emails are and the Director himself notes they may not even be significant," Podesta wrote. "It is extraordinary that we would see something like this just 11 days out from a presidential election."
Clinton is also calling on the FBI to release more information about its review of the emails.
She says "Let's get it out."
The Democratic presidential candidate says the American people deserve to have as much information as possible before they vote Nov. 8.
Clinton says she's confident investigators won't find information that would cause the FBI to change its decision to close the investigation without filing charges in July.
Clinton spoke hours after the FBI announced it was reviewing newly discovered emails to see if they are relevant to its closed investigation into her private email server.
"Donald Trump says he can still win. And he's right - anything can happen in an election," she said.
Republican presidential candidate Donald Trump pounced on the news at a rally in Manchester, New Hampshire.
"Hillary Clinton's corruption is on a scale we have never seen before. We must not let her take her criminal scheme into the Oval Office," Trump said. "This was a grave miscarriage of justice that the American people fully understood. Perhaps finally justice will be done."
Trump said he had "great respect" for the fact that the FBI and the Department of Justice are now "wiling to have the courage to right the horrible mistake that they made" in concluding the investigation earlier.
Republican Rep. Robert Goodlatte, who is the chairman of the House Judiciary Committee, issued a statement in response on Twitter.
The news comes nearly four months after Comey announced that no charges would be filed against Clinton over her use of a private email server while she was Secretary of State.
Clinton's email use has been one of the biggest vulnerabilities in her campaign for the White House. Even if she wins, Republicans have vowed the issue will follow her, promising continuing investigations.
But her campaign was fending off other political problems as well, still trying dismiss the revelations in thousands of messages stolen from the private account of a top Clinton aide, part of a hack the Democratic campaign has blamed on the Russians.
Correspondence made public on Wednesday showed longtime Bill Clinton aide Doug Band describing overlapping relationships of the Clintons' global philanthropy and the family's private enrichment.
"These are illegally stolen documents," Clinton campaign manager Robby Mook told reporters on her campaign plane. "We're not going to spend our campaign fighting back what the Russians want this to be about."
ABC NEWS: Timeline of the Hillary Clinton email saga
So far, the email-related controversies haven't seemed to hurt her campaign in the final weeks.
Recent surveys show Clinton retaining her lead in national polls and making gains in some swing states. Her campaign announced plans to hold a rally in Arizona next Wednesday, a traditionally red state put in play by Trump's deep unpopularity among minority voters, Mormons and business leaders.
With information from ABC News and The Associated Press