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    FBI probing new Clinton emails

    Costa: This is a 'curve ball' in the political race
    Costa: This is a 'curve ball' in the political race   

    The FBI is probing new emails related to Hillary Clinton, FBI Director James Comey said in a Friday letter.

    "In previous congressional testimony, I referred to the fact that the Federal Bureau of Investigation (FBI) had completed its investigation of former Secretary Clinton's personal email server. Due to recent developments, I am writing to supplement my previous testimony," 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.

    "Although 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, I believe it is important to update your Committees about our efforts in light of my previous testimony," Comey concluded.

    Stocks turned negative after the report of the new probe. Many analysts have said that markets were pricing in a Clinton victory in November.

    The letter was addressed to several lawmakers who chaired committees relevant to the FBI investigation into Clinton's use of a private email server.

    A representative for the Clinton campaign told NBC News that the team had "no idea" what Comey was referencing in his Friday letter. The State Department declined to comment to Reuters.

    Comey had previously announced that his department was not recommending that the Justice Department bring charges against Clinton for her handling of classified information on a private server. In that July speech, Comey had insisted the decision was made without outside influence and that it was the professional judgement of the FBI which led to his announcement.

    "Although there is evidence of potential violations regarding the handling of classified information, our judgment is that no reasonable prosecutor would bring such a case," Comey said at the time.

    FBI Director James Comey.
    Saul Loeb | AFP | Getty Images
    FBI Director James Comey.

    That announcement came as a relief to Democrats who feared that charges against their party's presidential nominee would stymie her efforts to win the White House.

    After a long campaign season, the November 8 elections were looking to go in Clinton's favor, according to most polling averages, but Friday's announcement could potentially offer a major political inflection point.

    Prominent Republicans took to Twitter to celebrate the news of the FBI probe.

    Donald Trump campaign manager Kellyanne Conway tweeted that "A great day in our campaign just got even better."

    For his part, Trump celebrated the news at a Friday campaign event.

    "They are reopening the case into her criminal and illegal conduct that threatens the security of the United States of America" Trump said to loud applause. "Hillary Clinton's corruption is on a scale we have never seen before, we must not let her take her 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, and it is everybody's hope that it is about to be corrected," Trump added.

    NBC News reported that Judiciary Committee Republicans were not expecting this news from the FBI, and it is not in response to any question from the Senate Judiciary Republicans.

    "We're interested to know what else they're going to be looking at," a spokesman for the Judiciary Committee Republicans said.

    "The FBI's decision to reopen its investigation into Secretary Clinton reinforces what the House Judiciary Committee has been saying for months: the more we learn about Secretary Clinton's use of a private email server, the clearer it becomes that she and her associates committed wrongdoing and jeopardized national security," House Judiciary Committee Chairman Bob Goodlatte (R-Va.) said in a statement. "Now that the FBI has reopened the matter, it must conduct the investigation with impartiality and thoroughness. The American people deserve no less and no one should be above the law."

    This story is developing. Please check back for further updates.

    —NBC News contributed to this report.