Updated 12:55 p.m.
FBI Director James Comey has notified members of Congress that the FBI has reopened its investigation into the handling of classified information in connection with Democratic presidential candidate Hillary Clinton'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 today, but had no further comment. The Justice Department says it will have no comment either.
Two sources familiar with the investigation also told Johnson that the matter dos 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.
The Clinton campaign has also not issued a comment yet. She is campaigning in Iowa Friday.
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."