The FBI is investigating whether new emails that may contain classified information have emerged in its probe of Hillary Clinton's private server.
The FBI said in July its investigation was finished, so this disclosure to the US Congress raises the possibility of the FBI reopening the criminal investigation.
In a letter sent on Friday to congressional leaders, FBI director James Comey said new emails have come to light recently that have prompted investigators to take another look at the sensitive government information that flowed through the private email server Clinton used while serving as secretary of state.
It was not clear from Comey's letter where the new emails came from or who sent or received them.
Perhaps unsurprisingly, Donald Trump has praised the FBI's decision which comes just days before the election. He said on Friday "perhaps finally justice will be served".
He was addressing a roaring crowd in Manchester, New Hampshire, shortly after news broke of the FBI decision.
Trump said that "Clinton's corruption is on a scale we have never seen before" and that "we must not let her take her criminal scheme into the Oval Office". Trump said he had "great respect" for the FBI's decision.
He had previously been very critical of the FBI and Department of Justice for the earlier decision not to bring charges against the Democratic nominee.
House of Representatives speaker Paul Ryan said Clinton has "nobody but herself to blame".
"She was entrusted with some of our nation's most important secrets and she betrayed that trust by carelessly mishandling highly-classified information," Ryan said in a statement.
"This decision, long overdue, is the result of her reckless use of a private email server and her refusal to be forthcoming with federal investigators.
"I renew my call for the director of national intelligence to suspend all classified briefings for secretary Clinton until this matter is fully resolved."
Clinton's team is yet to respond.