According to a letter written to various congressional committees on Friday, Comey said the FBI has learned of the existence of emails that appear to be pertinent to the investigation.
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Comey said he learned of this on Thursday and has agreed that the FBI should review the emails and determine if they contain classified information.
Comey's letter did not specify where the new emails came from or who sent or received them, but a U.S. official told the Associated Press that the emails related to Clinton's investigation came from the sexting probe regarding Anthony Weiner.
The FBI said in July its investigation was finished.
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"Although the FBI cannot yet assess whether or not this material may be significant, and I cannot predict how long it will take us to complete this additional work, I believe it is important to update your Committees about our efforts in light of my previous testimony," the letter stated.
John Podesta, chairman of Clinton's presidential campaign, issued a statement in which he called on Comey to "provide the American public more information than is contained in the letter he sent to eight Republican committee chairmen."
Podesta insisted that the FBI director's words were not consistent with characterizations that the agency was "reopening" an investigation.
Comey's letter "refers to emails that have come to light in an unrelated case," Podesta stated, "but we have no idea what those emails are and the Director himself notes they may not even be significant."
It appears that unrelated case is the Anthony Weiner sexting investigation. Weiner is divorcing from Huma Abedin, one of Clinton's closest advisors.
During a press conference Friday afternoon, Clinton addressed the investigation and called on the FBI to release more information about the emails that may be related to its investigation into her private server.
She said she is confident investigators will not find information that would cause the FBI to change its decision to close the investigation without filing charges in July.
The new development came 11 days before Nov. 8's election, and several prominent Republicans welcomed the move by the FBI.
"Hillary Clinton's corruption is on a scale we have never seen before," Donald Trump said at a rally in New Hampshire on Friday. "We must not let her take her criminal scheme into the Oval Office."
The GOP presidential candidate described the bureau's handling of its initial inquiry into Clinton's email server as "a grave miscarriage of justice that the American people fully understood."
"Perhaps finally justice will be done," he added.
NEW: Paul Ryan calls for Director of National Intelligence to suspend classified briefings for Clinton after FBI reopens email investigation pic.twitter.com/kaVI3Gb3a7— ABC News (@ABC) October 28, 2016