The FBI on Friday said it is assessing new emails "pertinent" to its investigation into Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE's use of a private email server, a stunning and unexpected move that comes more than a week before the presidential election.

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In a letter sent to lawmakers on Friday, FBI Director James Comey said the bureau has learned of the existence of more emails "that appear to be pertinent to the investigation." The messages were found "in connection with an unrelated case," Comey wrote without further explanation.

Law enforcement officials told The New York Times that the emails were uncovered after the FBI seized devices belonging to longtime Clinton aide Huma Abedin and her estranged husband, Anthony Weiner, who is under investigation for allegedly sending sexually explicit messages to an underage girl.

After being briefed by his team, Comey "agreed that the FBI should take appropriate investigative steps" to determine whether the emails "contain classified information, as well as to assess their importance to our investigation."

Comey said he could not predict how long it would take the bureau to assess whether the new emails are "significant," meaning the investigation could hang over Clinton's head through the election on Nov. 8.

BIG: The FBI is reopening its investigation into @HillaryClinton's email server. Here's the letter from the FBI to Congress: pic.twitter.com/OKjipTeiJp — Frank Thorp V (@frankthorp) October 28, 2016

The FBI’s surprise announcement jolted the presidential race, thrusting the controversy over Clinton’s use of a private email system for government business back into the political spotlight with just 11 days until Election Day.

Clinton's Republican opponent, Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE, has attacked her over the server, arguing it should disqualify her from being president. He seized on the FBI's decision at a rally on Friday afternoon as he seeks to make up ground in the polls.

"Hillary Clinton's corruption is on a scale we have never seen before," Trump said to thunderous chants of "lock her up" in Manchester, N.H.

"We must not let her take her criminal scheme into office. I have great respect for the FBI and Department of Justice are now willing to have the courage to right the horrible mistake they made."

Republicans and other critics of Clinton jumped on the timing of the announcement as an indicator that the FBI had found serious evidence that she or her aides committed a crime.

“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 Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (R-Va.) said in a statement.

Libertarian Party presidential nominee Gary Johnson Gary Earl JohnsonWhat the numbers say about Trump's chances at reelection Presidential race tightens in Minnesota as Trump plows resources into state The Hill's Campaign Report: Biden condemns violence, blames Trump for fomenting it l Bitter Mass. primaries reach the end l Super PAC spending set to explode MORE told CNN, “There has to be something there.”

Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) in a statement said Clinton should be denied the classified intelligence briefings that are given to presidential nominees.

“Yet again, Hillary Clinton has nobody but herself to blame. She was entrusted with some of our nation’s most important secrets, and she betrayed that trust by carelessly mishandling highly classified information," Ryan said in a statement.

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

Yet Comey's brief letter Friday left many things unanswered, including who sent the emails.

A senior law enforcement official told NBC News that the letter was sent to Capitol Hill out of “an abundance of caution.”

Some questioned the timing of the announcement and said the FBI owes the public a fuller explanation.

"Director Comey should give a more complete explanation. Is this reviewing newly found emails? Is this reopening? Too much at stake," tweeted John Weaver, who worked on the presidential campaign of Ohio Gov. John Kasich (R).

The FBI in July said it had completed its yearlong probe into whether the former secretary of State mishandled classified information on her personal server. The investigation was never formally closed from an administrative standpoint.

Comey at the time called Clinton "extremely careless" but declined to recommend criminal charges to the Justice Department, outraging Republicans who said she should have been indicted.

The FBI director repeatedly said Clinton's use of the server did not rise to the level of criminal behavior because investigators were unable to amass any evidence that she had intended to mishandle classified information. The probe did reveal that some information marked classified at the time passed through her servers.

Clinton has apologized for her use of the private setup, calling it a "mistake."

The FBI investigation has been under fierce scrutiny from lawmakers, Trump and conservative groups, who believe Clinton received preferential treatment from the Justice Department under President Obama's administration.

The bureau has made public hundreds of pages of documents related to the probe, including the official summaries of interviews done by investigators, and the State Department is releasing batches of emails uncovered during the investigation.

Critics of the Justice Department's decision not to charge Clinton in the case have seized on individual revelations in these documents as evidence that the investigation was mishandled.

Earlier on Friday, the conservative watchdog group Judicial Watch announced a lawsuit against the FBI, citing "an air of corruption" and seeking records related to its work.

Comey has defended the integrity of the probe.

“You can call us wrong, but don’t call us weasels. We are not weasels,” Comey said at a House Judiciary Committee hearing in September. “We are honest people, and whether or not you agree with the result, this was done the way you want it to be done."

NBC News first reported the existence of Comey's letter on Friday.

The full text of Comey's letter reads:

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. I am writing to supplement my previous testimony.

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 agree that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether the contain classified information, as well as to assess their importance to our investigation.

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 the additional work, I believe it is important to update your Committees about our efforts in light of my previous testimony.

Updated at 2:24 p.m.