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WASHINGTON -- FBI Director James Comey told lawmakers Friday the bureau is reopening the investigation into Hillary Clinton's personal email server, a surprise development 11 days ahead of the election.

After recommending earlier this year that the Department of Justice not press charges against the former secretary of state, Comey said in a letter to eight congressional committee chairmen that "recent developments" urged him to take another look.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear pertinent to the investigation," Comey wrote the chairmen. "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."

Comey said that he was not sure how long the additional review would take and said the FBI "cannot yet assess whether or not this material may be significant."

Law enforcement sources say the newly discovered emails are not related to WikiLeaks or the Clinton Foundation. They would not describe in further detail the content of the emails. It's also unclear whether the emails in question are from Clinton herself.

Clinton's campaign learned of the news while they were aboard a flight to Iowa.

"We're learning about this just like you all are," a Clinton aide said.

The surprising news jolts a presidential race that had largely settled as Republican presidential nominee Donald Trump struggled in national and key battleground polls.

Now, Clinton will be placed back on the defensive and forced to confront yet again questions about her trustworthiness.

Trump pounced on the news at the opening of his Friday afternoon rally in Manchester, N.H.

"Hillary Clinton's corruption is on a scale we've never seen before," Trump said. "We must not let her take her criminal scheme into the oval office."

Trump's campaign manager Kellyanne Conway tweeted after the news broke, "A great day in our campaign just got even better."

House Speaker Paul Ryan said Friday that Clinton betrayed Americans' trust for handling "the nation's most important secrets."

"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," Ryan said in a statement. "I renew my call for the Director of National Intelligence to suspend all classified briefings for Secretary Clinton until this matter is fully resolved."

The Department of Justice declined to comment Friday.

Despite lashing Clinton's email practices as "extremely careless," Comey declined earlier this summer to suggest prosecution. That move was instantly was lambasted by Republicans -- some of whom decried the department's politicization. Comey eventually was called to Capitol Hill to testify and defend the FBI's integrity and decision process.