Updated at 3:56 p.m.
Newly discovered emails being examined by the FBI in relation to Hillary Clinton's email server came to light in the course of an unrelated criminal investigation of Anthony Weiner, a source familiar with the matter tells NPR's Carrie Johnson.
Weiner is the estranged husband of close Clinton aide Huma Abedin; he has been under scrutiny for sending illicit text messages to an underage girl.
Earlier Friday FBI Director James Comey notified members of Congress that the FBI had reopened its investigation into the handling of classified information in connection with the Democratic presidential candidate's use of a private email server while secretary of state.
In a letter to the leaders of congressional oversight committees, Comey wrote: "In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. 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."
He added, "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."
Earlier this year, Comey closed the yearlong investigation without filing any charges against Clinton and her associates.
The FBI confirmed to NPR's Carrie Johnson that the letter to oversight committees went out Friday, but had no further comment. The Justice Department says it will have no comment either.
Two sources familiar with the investigation also told Carrie that the matter does not relate to the WikiLeaks email release, which has been releasing hacked emails from Clinton campaign Chairman John Podesta's account. The newly discovered emails were not withheld by the Clinton campaign, the sources said, and it was not clear whether Clinton sent or received anything in the new pool of documents.
Podesta issued a statement Friday afternoon calling on Comey to release more information about the reopened investigation, especially given its timing in relation to the election:
"FBI Director Comey should immediately provide the American public more information than is contained in the letter he sent to eight Republican committee chairmen. Already, we have seen characterizations that the FBI is 'reopening' an investigation but Comey's words do not match that characterization. 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.
"It is extraordinary that we would see something like this just 11 days out from a presidential election.
"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."
State Department spokesman Mark Toner says his agency has not received a letter from the FBI that he is aware of. He says the department learned of the letter from news reports, per NPR's Michele Kelemen, and he doesn't know whether the emails are from her time at State.
Republicans quickly took the opportunity to renew criticism of Clinton's use of a private email server.
Donald Trump spoke about the story shortly after it broke at his rally in Manchester, N.H. "Hillary Clinton's corruption is on a scale we have never seen before," he said. "We must not let her take her criminal scheme into the Oval Office."
The Republican National Committee said the FBI's decision ahead of the election "shows how serious this discovery must be" and "raises serious questions about what records may have been turned over and why."
House Speaker Paul Ryan said in a statement that he is renewing his call for the director of national intelligence to suspend classified briefings for Clinton "until this matter is fully resolved."