"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Comey's letter said, referring to the FBI's previous investigation into Clinton's server. "I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation."
Republicans were quick on to jump on the news to bash the Democratic nominee. The Clinton campaign, meanwhile, called upon the FBI to provide more information about the new emails.
"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. "The Director owes it to the American people to immediately provide the full details of what he is now examining. We are confident this will not produce any conclusions different from the one the FBI reached in July."
Though Republicans and some news outlets were quick to use the letter to describe the investigation as being "reopened," NBC News' Pete Williams reported that it was not in fact technically closed, citing "evidentiary matters" and Freedom of Information Act issues that are still being resolved.
On the question of whether the newly-surfaced emails had been withheld by Clinton's staff, Williams said that "senior officials who are familiar with the thinking behind this letter" had told him they did not see evidence as such.
"This is not a matter, they say, it doesn't appear that the campaign or the Clintons or the State Department had emails that they didn't give to the FBI and that the FBI somehow found them some other way," Williams said. "It's not like that, they say."
The New York Times and the AP reported that the emails were uncovered after the FBI seized electronic devices owned by Clinton aide Huma Abedin and her estranged husband, former Rep. Anthony Weiner (D-NY). The devices were seized as part of its investigation into Weiner's alleged sexting with a 15-year-old girl. They contained emails from Clinton's private email server, according to the New York Times. NBC News reported that the device in question was Weiner's laptop, which Abedin also used to exchange emails with Clinton.
The New York Times also reported that the new emails "numbered in the thousands," according to a federal official.
Comey announced in July that the FBI would not recommend that charges be brought against Clinton for her use of the private email server, saying the decision to use it was "careless" but that she and her staff engaged in no “intentional misconduct,” according to the agency's investigation.
Republicans quickly pounced on the decision not to recommend charges, with GOP nominee Donald Trump calling it a "bad judgment." Congressional GOP lawmakers have signaled they intend to make investigating Clinton's years at the State Department a priority if she is elected President, including the private email server and her congressional testimony about it.
The FBI opened its probe into Clinton's email server in July of 2015, focusing on whether classified information was stored or disseminated on the private server -- an unclassified system -- in violation of federal law, or whether classified information was compromised by an intrusion into the server. The probe stemmed from the revelation of the private server as part of the investigations of the House Select Committee on Benghazi.
The FBI prove included an examination of some 30,000 emails that Clinton turned over to the State Department, as well as interviews with members of her staff and Clinton herself. The agency concluded that, while the server had some vulnerabilities to cyber intrusion, it had found no evidence that the server had in fact been compromised by cyber attack. It also concluded that there was no evidence that Clinton or staff intentionally deleted her private server emails as part of a cover-up.
In his letter Friday, Comey said that the "FBI cannot yet assess whether or not this material may be significant," nor could he predict how long it would take the agency to examine them.
Read the full letter below:
Correction: Due to an editing error, the original version of this story reported that the new emails were from Clinton's private server, which goes beyond what Comey's letter states. Reporting that has come out since has suggested that emails from Clinton's private email server were found on devices owned by Anthony Weiner and Human Abedin seized by the FBI.