The FBI on Friday dropped a bombshell on Hillary Clinton’s campaign less than two weeks before Election Day, with director James Comey announcing that the agency is reviewing new evidence in its investigation into her use of a private email server as secretary of state.

In a letter to several congressional committee chairmen, Comey wrote that, “In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to this investigation.”

Comey said he was briefed on those emails on Thursday and that he “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 did not specify where the additional emails came from.

Comey wrote that the FBI does not yet know if the new material is “significant” and did not provide a timeframe for investigating in the letter, which overall contained sparse details.

In July, Comey said the FBI was not recommending charges against Clinton, saying “no reasonable prosecutor” would bring such a case. But he did chastise her for being “extremely careless” in her handling of sensitive information. The controversy over Clinton’s use of the server, reported for the first time by The New York Times in March 2015, has dogged her presidential run since its beginning.

While Clinton said her decision to use a private email server was a “mistake,” she has steadfastly said that she violated no laws.

The news broke as Clinton was en route to a campaign event. As she got off the plane, she smiled and waved — and ignored all questions by the press around her.

But her campaign appeared blindsided by the development. When asked by NBC News to respond to the revelation, a top Clinton campaign spokesperson said, «No idea.»

Clinton’s running mate, Tim Kaine dodged when asked about the FBI investigation during a campaign stop in Florida. “I’ve got to read more. I’ve got to read a little more,” he said.

Donald Trump, who has often egged on his crowds as they chant “lock her up,” immediately celebrated the FBI’s move.

«I need to open with a very critical, breaking news announcement,» Trump said at the start of a rally in New Hampshire. «Hillary Clinton’s corruption is on a scale that we have never seen before. We must not let her take her are criminal scheme into the Oval Office. I have great respect for the fact that the FBI and the department of justice are now willing to have the courage to right the horrible mistake that they made.»

«This was a grave miscarriage of justice that the American people fully understood,» he continued. «And it is everybody’s hope that it is about to be corrected.»

Trump’s campaign manager, Kellyanne Conway, also cheered the news. «A great day in our campaign just got even better. FBI reviewing new emails in Clinton probe,» Conway tweeted.

A spokesman from the Senate Judiciary panel said they only learned of the new evidence on Friday from Comey’s letter. They were unsure what emails the FBI had discovered and did not know what the “unrelated case” pertained to. However, the aide said the panels will likely seek to find out more in the coming days.

NBC News justice correspondent Peter Williams, citing senior officials familiar with the thinking behind Comey’s letter, reported Friday afternoon that investigators came across “some device” that led them to some other emails.

The officials said the “unrelated case” was not the probe into Russia’s alleged hacking of U.S. campaign targets, including Clinton campaign chairman John Podesta’s private email, Williams reported. He also said officials do not believe this is a matter of Clinton or her aides withholding evidence, and that the emails are not messages from Clinton herself.

The officials added that there is no way the matter will be resolved within 11 days.

The revelation, the latest October surprise, has the potential to change the dynamic of the race, just as Clinton had been pulling away from Trump in recent weeks. Trump was hit by his own surprise on Oct. 7 with the release of a 2005 “Access Hollywood” tape in which the billionaire was caught boasting about being able to get away with grabbing women by the genitals because he’s “a star.”

Republicans, many of whom had become increasingly resigned to the idea that Clinton was running away with the election, appeared buoyed by the FBI’s decision to take another look at Clinton’s handling of classified materials.

House Speaker Paul Ryan called the FBI’s move «long overdue.» «Yes again, Hillary Clinton has nobody but herself to blame,» he said in a statement. «She was entrusted with some of our nation’s most important secrets, and she betrayed that trust by carelessly mishandling highly classified information.»

House Judiciary Committee Chairman Bob Goodlatte said it is clear Clinton and her aides committed wrongdoing.

“Now that the FBI has reopened the matter, it must conduct the investigation with impartiality and thoroughness,” he said. “The American people deserve no less and no one should be above the law.”

The Republican National Committee released a statement early Friday afternoon, saying the FBI’s decision to release the news just 11 days before the election shows how serious the discovery must be.

“This alone should be disqualifying for anyone seeking the presidency, a job that is supposed to begin each morning with a top secret intelligence briefing,” RNC Chairman Reince Priebus said.

The FBI investigation began in July 2015, when the Justice Department received an official referral from the Intelligence Community Inspector General about the potential compromise of classified information in her account.

Agents spent months trying to obtain all the devices that might have held Clinton’s email correspondence. In short order, they obtained thumb drives from Clinton’s lawyers, a back-up server from a Colorado firm that handled Clinton’s email system after she left the State Department. They also contacted the State Department, her former aides, and other former government officials who were in correspondence with her.

Eventually, the FBI found more than 10,000 email messages they believed could be work-related and at least somewhat different than the ones Clinton turned over to State in December 2014. However, State officials have said many of the newly-found messages are identical or nearly identical to emails State already had from Clinton or elsewhere.

While Comey has maintained the Clinton email investigation was handled similarly to others into mishandling of classified information and that the same standards were applied to Clinton and her aides as in other cases, he has acknowledged being more public in describing the FBI’s conclusions.

In early July of this year, Comey made a highly unusual press statement announcing that the FBI wasn’t recommending any charges in connection with the Clinton email probe. He also made a pair of appearances on Capitol Hill, engaging in contentious exchanges with Republican lawmakers who said Comey’s decision defied logic.

Many GOP House members said there was ample basis to charge Clinton with “gross negligence” in handling classified information, but the FBI director said that charge has been brought only once in nearly 100 years in a case involving an FBI agent who regularly left classified information in an open briefcase while having an affair with a woman working as a double agent for the U.S. and China.

Annie Karni, Louis Nelson and Rachael Bade contributed to this report.