The FBI is probing new emails related to Hillary Clinton, FBI Director James Comey said in a Friday letter to lawmakers.
Those new emails were discovered during an FBI investigation in which the devices of former Congressman Anthony Weiner and Clinton aide Huma Abedin were seized, NBC News confirmed. The New York Times first reported the news.
While examining Weiner's laptop, investigators discovered Abedin also used the laptop, which contained some emails between Abedin and Clinton, NBC News reported.
"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 in the letter addressed to several lawmakers who chaired committees.
In a letter addressed to Comey and Attorney General Loretta Lynch, Democratic congressmen Elijah Cummings and John Conyers called for full disclosure from the Department of Justice and the FBI "in order to debunk these conspiracy theories and correct the public record."
Stocks turned negative after the report of the new probe. Many analysts have said that markets were pricing in a Clinton victory in November.
Clinton campaign chair John Podesta said in a statement, "it is extraordinary that we would see something like this just 11 days out from a presidential election."
"The Director owes it to the American people to immediately provide the full details of what he is now examining. We are confident this will not produce any conclusions different from the one the FBI reached in July," Podesta said in a statement.
The State Department declined to comment to Reuters.
NBC News reported that officials familiar with Comey's letter said it did not appear that anyone had intentionally withheld the new evidence. The letter was sent to lawmakers "out of an abundance of caution," a senior law enforcement official told NBC News. The official also told NBC News that these emails were not held by someone who was investigated in the Clinton email case.
NBC reported that the emails were not revealed by hackers or in the Wikileaks cache related to Clinton's team. The newly discovered emails did not come from her private email server, the Associated Press reported citing a U.S. official.
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.
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.
Even if Clinton were to win on election day, the pall of an ongoing investigation would likely weigh on the president-elect.
The White House said, however, that nothing has happened that would change President Barack Obama's view of Clinton.
"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.
Indiana Gov. Mike Pence commended the FBI's actions and called on the agency to "immediately release all the emails pertinent" to the investigation. He said at a rally in Pennsylvania that the American people have a right to know before they vote.
Trump campaign manager Kellyanne Conway tweeted that "a great day in our campaign just got even better."
Other prominent Republicans took to Twitter to celebrate the news of the FBI probe.
The FBI's July announcement that it would not recommend charges created instant backlash from congressional Republicans, who said they felt Comey had laid out a sufficient basis for criminal charges.
"The FBI's recommendation is surprising and confusing. The fact pattern presented by Director Comey makes clear Secretary Clinton violated the law," House Oversight Committee Chairman Jason Chaffetz, a Utah Republican, said in a release at the time. "Individuals who intentionally skirt the law must be held accountable."
Trump referenced that outlook Friday, saying that the system "might not be as rigged as I thought. I think they're going to right the ship."
NBC News reported that Judiciary Committee Republicans were not expecting Friday's news from the FBI, and it was 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."
Rep. Darrell Issa, R-Calif., said in a statement that Friday's news is "exactly why we need strong watchdogs in Congress to ensure thorough oversight of the executive branch."
"The federal government constantly needs to be held accountable to curb poor judgment — like using a private server to circumvent federal records laws — and incomplete investigations that fail to deliver justice and erode public faith in government," said Issa, a senior member of the House Judiciary Committee. "This decision also makes clear that Hillary Clinton's continued efforts to deceive the public on her actions that willingly put classified information at risk, need to come to an end."
Republican National Committee Chairman Reince Priebus said in a statement that the FBI's letter coming just 11 days before the election "shows how serious this discovery must be."
"This stunning development raises serious questions about what records may not have been turned over and why, and whether they show intent to violate the law. What's indisputable is that Hillary Clinton jeopardized classified information on thousands of occasions in her reckless attempt to hide pay-to-play corruption at her State Department," Priebus said in a statement. "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."
This story is developing. Please check back for further updates.
—NBC News contributed to this report.