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By Mark Hosenball and Jonathan Allen

WASHINGTON/NEW YORK (Reuters) - The FBI is investigating more emails as part of a probe into Hillary Clinton's use of a private email system, it said on Friday, in a new twist that could damage the Democratic candidate in the 2016 presidential race.

Federal Bureau of Investigation Director James Comey said in a letter to senior lawmakers that the agency would determine whether the additional emails contained classified information, adding that he did not know "how long it will take us to complete this additional work."

The announcement came as Clinton and Republican opponent Donald Trump enter the final stretch of campaigning ahead of the Nov. 8 election.

In a news conference late on Friday in Des Moines, Iowa, Clinton urged Comey to release more details about what the FBI was looking for in the newly discovered emails.

She leads Trump in opinion polls after a bruising campaign in which she has struggled to convince voters that she is trustworthy and honest. Fresh revelations about her use of email are unlikely to assuage those concerns, and questions around the FBI investigation will now likely dog her in the coming days as she campaigns across battleground states.

U.S. stocks immediately fell sharply on the news, but went on to partially recover.

The FBI spent about a year investigating Clinton's use of the unauthorized server at her home in Chappaqua, New York, while she was U.S. secretary of state after classified government secrets were found in some of her emails.

Comey said in July that while "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."

Although Comey recommended no criminal charges be brought against Clinton, Trump has repeatedly said her email practices are criminal and should disqualify her for office. He seized on Friday's development at rallies in Maine and New Hampshire.

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"This is the biggest political scandal since Watergate, and I'm sure it will be properly handled from this point forward," Trump told a crowd in Lisbon, Maine.

"We hope that all, all justice will be fully served," he said. Supporters cheered his words and chanted, "Lock her up."

Clinton said she had learned of the newly discovered emails from news reports.

"I'm confident whatever they are will not change the conclusion reached in July," she said. "That’s why it’s incumbent upon the FBI to tell us what’s going on."

WEINER

Two sources close to the investigation said the latest emails were discovered not during an investigation into Clinton, but rather as part of a separate probe into Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin.

The FBI has been investigating illicit text messages allegedly sent from Weiner to a 15-year-old girl in North Carolina, and found the Clinton emails on a device related to that investigation, the sources said.

Abedin told federal investigators in April that she used several email accounts for her work, including a Yahoo email, according to a summary of the interview released by the FBI in September. She said it was difficult to print from the State Department's email system so she routinely forwarded documents to her private accounts when she needed to print them out, according to the summary.

Abedin announced her separation from Weiner in August after a sex scandal similar to an earlier incident that led him to resign from the U.S. Congress.

Lawyers representing Abedin did not respond to questions sent by email on Friday. Weiner did not immediately respond to an email seeking comment, and he did not respond to phone calls.

Hank Sheinkopf, a Democratic strategist who had previously advised former President Bill Clinton but has no role with Hillary Clinton's campaign, said the linking of Clinton's email woes with Weiner's sex scandals made it harder for her campaign to distinguish itself from Trump's sex scandals.

"The whole campaign is now smeared with sex, corruption and scandal," he said. "Nobody remembers the beginning of something, they only remember the end. What are they going to remember? They're all the same: sex, scandal, corruption, emails. People are going to have trouble sorting out all this information."

"EXTRAORDINARY"

Clinton's campaign chairman, John Podesta, said it was "extraordinary" for the FBI to release the letter so close to the end of a hotly contested election.

"The Director owes it to the American people to immediately provide the full details of what he is now examining," Podesta said in a statement. "We are confident this will not produce any conclusions different from the one the FBI reached in July."

Clinton has repeatedly apologized for using the private email server in her home instead of a government email account for her work as secretary of state from 2009 to 2013. She has said she did not knowingly send or receive classified information.

While the FBI probe of new emails will likely prove a distraction to Clinton in the coming days, it was unclear what impact it would have when Americans go to the polls. Election experts say about 20 percent of ballots have already been cast, as more Americans vote by mail or go to the polls early.

"A lot of concern about the emails has already been baked into this electoral cake I think," said Linda Fowler, a professor of government at Dartmouth College. "They know she did it, they know it was inappropriate and, failing some sensational revelation on Nov. 6, it's hard to see that it's going to make that big a difference."

Still, Republican lawmakers, who are facing a difficult fight to keep their majorities in both the U.S. Senate and the House of Representatives, leaped to condemn Clinton.

House Speaker Paul Ryan reiterated his call that the Democratic nominee be barred from briefings involving classified information until the investigation is over. Reince Priebus, the head of the Republican National Committee, said the FBI's decision to look into the emails shows "how serious this discovery must be."

Clinton did not respond to reporters' shouted questions about the news when she left her plane for a campaign stop in Cedar Rapids, Iowa.

U.S. stocks declined in a volatile session on Friday but partially recovered from the sharp drop spurred by the FBI announcement. The Dow Jones Industrial Average closed the day down about 9 points, or .05 percent, while the benchmark 10-year Treasury note was unchanged on the day. The dollar also fell against major currencies.

(Additional reporting by Susan Heavey, Julia Harte and Andy Sullivan in Washington, Steve Holland in Manchester, New Hampshire, Jeff Mason in Des Moines, Iowa, and Sam Forgione and Nate Raymond in New York; Writing by Jonathan Allen, Patricia Zengerle and Emily Flitter; Editing by Jonathan Oatis, Alistair Bell and Lisa Shumaker)