The FBI will again be looking into Hillary Clinton's email use as secretary of state, bureau director James Comey said in a letter to the Senate Judiciary Committee on Friday.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," he wrote. "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 clarified the FBI cannot yet assess whether or not the material may be significant, nor can he predict how long the investigation will take. The New York Times reported the emails referenced were uncovered after the FBI seized devices belonging to Clinton aide Huma Abedin and her estranged husband, Anthony Weiner. 

U.S. stocks retreated after the announcement, seemingly reflecting increased concern regarding such a big revelation shortly before the election. The S&P 500 was down about .53%, the Nasdaq composite index dipped .56% while the Dow Jones Industrial average was down .16% midday. 

The S&P 500 fell 17.9 points to 2122.37 from 2140.27 from 1:00 p.m. ET around when the news broke to 2:00 p.m. an hour later. The Nasdaq dipped 39.38 points to 5187.94 to 5227.32.

The FBI closed its investigation into Clinton's email use at the State Department in July. Comey called Clinton's team "extremely careless" in its handling of classified information at the time but did not recommend the Department of Justice make any charges against Clinton or any of her aides.

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

Clinton campaign chairman John Podesta released a statement on the letter and called upon Comey to "immediately provide the American public more information." He said the Clinton camp has "no idea" what the emails are and that they are confident this will not produce any conclusions different from those reached in July. 

"It is extraordinary that we would see something like this just 11 days out from a presidential election," he said. 

Donald Trump reacted to the news in a statement of his own.

"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," he said. "I have great respect for the fact taht the FBI and the DOJ are now willing to have the courage to right the horrible mistake they made. This was a grave miscarriage of justice that the American people fully understand. It is everybody's hope that it is about to be corrected."

Speaker of the House Paul Ryan pounced on the news, calling the FBI's decision "the result of her reckless use of a private email server, and her refusal to be forthcoming with federal investigators." He renewed a call for intelligence briefings to be suspended until the matter is resolved.

Utah Representative Jason Chaffetz, the Republican chairman of the House Committee on Oversight and Government Reform who has led the charge against Clinton, reacted to the news on Twitter.

FBI Dir just informed me, "The FBI has learned of the existence of emails that appear to be pertinent to the investigation." Case reopened

— Jason Chaffetz (@jasoninthehouse) October 28, 2016

Chaffetz had already promised to spend the next several years investigating Clinton. 

Comey's Friday letter comes just days ahead of the Nov. 8 election and is certainly unwelcome by Clinton and her supporters.