The FBI said Friday it would reopen its investigation into Hillary Clinton's private email server after learning of "the existence of emails that appear to be pertinent to the investigation" — emails that officials later said were discovered amid an investigation into former congressman Anthony Weiner.

FBI Director James Comey, in a letter to congressional leaders, said the team in charge of looking into Clinton's server briefed him Thursday on new emails it found "in connection with an unrelated case."

"The FBI cannot yet assess whether or not this material may be significant," Comey wrote. "I cannot predict how long it will take us to complete this additional work."

Here's the full letter:

FBI Letter by Brett LoGiurato on Scribd

The emails were uncovered after the FBI seized devices belonging to Huma Abedin and her husband, Weiner, The New York Times reported, citing law enforcement officials. Prosecutors issued a subpoena for Weiner's cellphone and other records in late September amid allegations that he had been sexting with a 15-year-old girl.

Abedin is one of Clinton's top aides. She and Weiner separated in late August after The New York Post reported that Weiner sent suggestive images and messages to a woman for more than a year starting in early 2015.

Multiple outlets reported on Friday that the number of emails uncovered through Abedin's and Weiner's devices was in the thousands. The emails involved were apparently sent and received by Abedin over a long period of time, and were found on a device Weiner shared with Abedin.

The developments threw a wrench into an already turbulent presidential campaign with just 11 days before Election Day, coming as Clinton has held leads over Republican nominee Donald Trump in national polls and surveys in key battleground states.

Clinton spoke at a rally on Friday after the news broke, but did not mention the FBI's announcement. At a press conference later on Friday evening, she implored the FBI to release more information about the newly discovered emails.

“The American people deserve to get the full and complete facts immediately,” Clinton told reporters.

Stocks fell sharply after the FBI's announcement.

Trump, who has said he would "lock Clinton up" over her email scandal, seized on the news at a rally Friday.

"Clinton's corruption is not on a scale we have ever seen," he told a rowdy crowd. "The FBI and the Justice Department now have the chance to right the horrible mistake that they made" in not previously recommending that Clinton be charged with criminal wrongdoing.

House Speaker Paul Ryan released a statement renewing his call for the director of national intelligence to suspend classified intelligence briefings for Clinton, who "was entrusted with some of our nation's most important secrets, and betrayed that trust by carelessly mishandling highly classified information," he said.

On Twitter, some political analysts and observers condemned the FBI's handling of the announcement.

"It's bad enough having the media do the 'raises questions,' substance-free innuendo thing. Worse when the FBI director does it. Disgraceful," Paul Krugman, the economist and New York Times columnist, wrote on Twitter. "Comey needs to provide full info immediately. Otherwise he has clearly made a partisan intervention, betraying his office."

"I have zero faith that anyone will handle this responsibly," former Obama speechwriter Jon Favreau wrote. "It's insane. [Comey] at least owes the country a press briefing — anything more than a vague letter."

"Director Comey should give a more complete explanation. Is this reviewing newly found emails? Is this reopening? Too much at stake," wrote John Weaver, the former campaign strategist for Ohio Gov. John Kasich.

The new emails were reportedly uncovered after the FBI seized devices belong to Huma Abedin and her husband, Anthony Weiner. Prosecutors issued a subpoena for Weiner's cellphone and other records in late September amid allegations that he had been sexting with a 15-year-old girl. AP Photo

Comey explained why he sent the letter to Congressional leaders in a separate letter to his employees on Friday, acknowleding that "in a brief letter and in the middle of an election season, there is significant risk of being misunderstood."

"We don’t ordinarily tell Congress about ongoing investigations, but here I feel an obligation to do so given that I testified repeatedly in recent months that our investigation was completed," Comey wrote. "I also think it would be misleading to the American people were we not to supplement the record."

Clinton's email scandal has dogged the Democratic presidential nominee for more than a year. In March 2015, she first admitted to exclusively using a private email account to send and receive work-related emails while she served as secretary of state. The controversy compelled her to hand over roughly 30,000 work-related emails to the State Department, which have been released in batches since last year.

But she deleted about 30,000 additional emails from her server that she said were "personal" in nature before handing it over to the FBI in August, five months after she gave individual emails to the State Department.

Democratic nominee Hillary Clinton. Thomson Reuters

After examining her emails for more than a year, the State Department issued a scathing report in May concluding that she "did not comply" with its policies when she chose to use a personal email account to conduct government business.

The State Department faulted Clinton and previous secretaries of state for poorly managing email and other computer information and for slowly responding to new cybersecurity risks.

Two months later, the FBI decided not to recommend that the Justice Department bring charges against Clinton, saying Clinton's conduct did not meet the threshold used to prosecute past violators who shared classified information over unclassified channels.

"In looking back at our investigations into mishandling or removal of classified information, we cannot find a case that would support bringing criminal charges on these facts," Comey said in July, after the FBI interviewed Clinton for three hours about her use of the server while at the State Department.

When asked during a congressional hearing in July if he would reopen the investigation if he "discovered new information that was both relevant and substantial," Comey replied that the FBI "would certainly look at any new and substantial information.”

FBI Director James Comey testifies during a hearing before House Oversight and Government Reform Committee July 7, 2016 on Capitol Hill in Washington, DC. Alex Wong/Getty Images Clinton's unusual email system was originally set up by a staffer during Clinton's 2008 presidential campaign. It replaced another private server used by her husband, Bill Clinton.

The new server was run by Bryan Pagliano, who had worked as the IT director on Hillary Clinton's campaign before she joined the State Department in May 2009. In 2013 — the same year she left the State Department — Clinton hired Platte River Networks, a small Denver-based IT firm, to oversee the system.

Clinton has since apologized for using the email server and insisted she never sent or received information marked "classified" using her private email account.

The news tops off a particularly turbulent week for the Clinton campaign in the campaign's final days.

It included the release of a hacked memo published by WikiLeaks titled "Bill Clinton, Inc." that raised questions about the blurring of lines between the Clinton Foundation charity and the Clintons' personal finances. The Obama administration, meanwhile, confirmed this week that health insurance premiums on the exchanges set up by the Affordable Care Act were set to increase significantly in 2017.