Updated at 7:58 p.m. ET
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. Sources said authorities seized electronic devices in their home, which led them to this new information.
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."
Clinton addressed the latest developments in a brief press conference Friday night in Des Moines, Iowa, saying it was "imperative that the bureau explain this issue in question — whatever it is — without any delay" with just 11 days to go until Election Day.
"We don't know the facts, which is why we are calling on the FBI to release all of the information that it has," Clinton said, noting that Comey himself "has said he doesn't know whether the emails referenced in his letter are significant or not."
As to reports that these emails surfaced during the Weiner investigation, Clinton said, "Right now, your guess is as good as mine, and I don't think that's good enough." She said neither she nor her campaign had been contacted by the FBI and they learned of the developments via news reports.
Earlier this year, Comey closed the yearlong investigation without filing any charges against Clinton and her associates.
The FBI confirmed to Carrie that the letter to the 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.
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 FBI request 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."