The announcement came as a shock to the Clinton campaign and sparked a flurry of swift reactions from the campaign trail to Wall Street.
Comey sent a letter to key members of Congress Friday informing them that although the FBI had previously announced the completion of its investigation into Clinton's email server, new information had prompted additional review.
"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Comey wrote in a letter released Friday.
He also said that he does not know how long the additional investigation will take.
Comey wrote that the FBI cannot yet assess whether the "material may be significant."
Comey and others have said repeatedly that while they found classified information on Clinton's server, they could not find evidence that Clinton knowingly sent or received sensitive information. Therefore, charges were not warranted, according to Comey.
It is unknown whether the latest emails could change any of the FBI's conclusions. The review is unlikely to be completed before Election Day, according to sources familiar with the investigation.
Why Is the FBI Reviewing This Case?
The emails that prompted the latest FBI review related to Clinton’s private server came from a separate federal investigation of former Rep. Anthony Weiner, multiple sources told ABC News.
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, emails were discovered on at least one device used by Weiner and his wife, Clinton aide Huma Abedin, according to sources familiar with the investigation.
Abedin was among the handful of people that had an email address on Clinton’s private server.
The FBI wants to sort through these emails, and they will determine whether the emails have any significant relevance to the Clinton probe and whether the emails contain any classified information.
These emails were not sent by Hillary Clinton, and the FBI has no evidence of wrongdoing by her, according to a source familiar with the investigation. 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.
Wasn't the Investigation Already Closed?
At his July 5 news conference where he first revealed the FBI's conclusions, Comey said, "After a tremendous amount of work over the last year, the FBI is completing its investigation and referring the case to the Department of Justice for a prosecutive decision."
Two days later, while testifying before a House panel on the FBI's investigation, Comey said this of his testimony: "I'm doing this 24 hours after the investigation closed."
Technically, this case wasn't closed because administrative actions were still underway, but Comey had publicly announced the end to the case.
Anytime information comes to the attention of law enforcement that may be relevant to even a closed case, investigators have an obligation to evaluate that information to see whether it is actually relevant. That a review of the emails is taking place does not mean they are necessarily significant to the Clinton probe.
Why Now, Just 10 Days Before the Presidential Election?
Once these emails were discovered and the decision was made to take a look at them for potential relevance, the FBI knew it was between a rock and a hard place, according to a source familiar with the matter. The FBI faced two decisions: Either disclose it after the election, and risk being accused of hiding it. Or disclose it now, and risk being accused of potentially influencing an election.
The FBI ultimately decided it was best to disclose the information as soon as possible and take their lumps in the name of transparency, according to the source.
What's the Reaction?
The Clinton campaign called on the FBI to "immediately" provide the public with more information than was in the letter to Congress.
"It is extraordinary that we would see something like this just 11 days out from a presidential election," Clinton campaign manager John Podesta said in a statement Friday.
Meanwhile, the Trump team delighted in the news. "A great day in our campaign just got even better. FBI reviewing new emails in Clinton probe," campaign manager Kellyanne Conway tweeted
Volatility on Wall Street spiked on this news and U.S. stocks swung from positive early today to negative.
Congress was also left stunned, but GOP members didn't waste time in criticizing Clinton, including fundraising off the FBI’s announcement.
"We are days out from Election Day and Hillary Clinton is once again under FBI investigation for potentially illegal activity on her private email server. Do you think Hillary Clinton can be trusted to serve as our Commander-in-Chief?" an email from Senate Republicans said.
Congressional Democrats have criticized the FBI for its handling of the case.
"The FBI has a history of extreme caution near Election Day so as not to influence the results. Today’s break from that tradition is appalling," said Sen. Dianne Feinstein, D-Calif., ranking member of the Senate Intelligence Committee.
But all sides of the political spectrum are calling on the FBI to release additional details.
ABC News’ Pierre Thomas, Cecilia Vega, Aaron Katersky, Rebecca Jarvis, Justin Fishel, Ben Siegel, Candace Smith and John Santucci contributed to this story.