An FBI spokesman today declined to elaborate and a spokesman for Attorney General Loretta Lynch declined to comment. A Clinton campaign spokesman did not immediately respond to a request for comment.
Comey provided no details about the unrelated case that resulted in the finding of the new emails. A law enforcement official, speaking on condition of anonymity, said the emails were "numerous" and said they are not related to hacks of recent Democratic party institutions or the emails that have been released in recent days by the anti-secrecy group WikiLeaks.
The official said once informed of the find, Comey felt an obligation to inform Congress, since he had previously told lawmakers the investigation had been completed. As a technical matter, however, the Clinton investigation was never formally closed, the official said.
When he announced the FBI's findings in July, Comey said that Clinton had been "extremely careless" in her handling of classified material, which had been found among the emails exchanged on her private server.
He had said that his investigators found evidence of potential violation of laws governing the handling of classified information, but found that "no reasonable prosecutor" would bring the case.
In particular, he said investigators did not find evidence that there had been intentional mishandling of classified material or indications of disloyalty to the US or efforts to obstruct justice.
Comey had come under enormous pressure from Republicans for his recommendation to bring no case against Clinton. Republican presidential candidate Donald Trump has repeatedly cited the decision as a sign of corruption endemic to Washington institutions and promised that, if elected, he would reopen the investigation.
Trump, addressing supporters in New Hampshire Friday, hailed the FBI's announcement - saying he had "great respect" for the agency's decision to "right the horrible mistake that they made."
"This is bigger than Watergate," Trump told the crowd.
"Perhaps, finally, justice will be done," he said, as the crowd pumped fists and cheered, "Lock her up! Lock her up!"
As the news broke, the Dow Jones industrial average dropped more than 150 points.
Word also began to spread quickly on Capitol Hill, where lawmakers saw the announcement as a potential game-changer for the election.
"A total bombshell," said Peter King, member of the House Homeland Security Committee. King predicted the FBI would not close its inquiry prior to the election, and said he believed Comey wanted the public to know of his move regardless of the outcome.
"He wants it all out there," King said.
WikiLeaks has been releasing emails hacked from the account of campaign chairman John Podesta in recent days, including material in which some of Clinton's closest advisers expressed surprise over her use of the server.
"[D]id you have any idea of the depth of this story?" Podesta asked campaign manager Robbie Mook late on March 2, 2015, the day the New York Times revealed Clinton had exclusively used a private account as secretary.
"Nope," Mook replied early the next day. "We brought up the existence of emails in [research] this summer but were told that everything was taken care of."
The State Department's deputy spokesman, Mark Toner, said the FBI has not notified them of the new emails and referred all questions to the FBI.
"We stand ready to cooperate if we're asked to do so," he told reporters. "But I don't have any additional details at this point."
