WASHINGTON/NEW YORK – The FBI says its investigating more emails as part of a probe into Hillary Clinton’s use of a private email system, in a new twist to the US presidential campaign that could damage the Democratic candidate just 11 days before Election Day.

Federal Bureau of Investigation Director James Comey said in a letter to Republican US congressional committee chairmen 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 November 8 election.

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.

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

FBI Director James Comey testifies before a House Homeland Security Committee hearing on Capitol Hill in Washington, US on July 14, 2016. (Photo: Reuters/ File Photo)

The FBI spent about a year investigating Clinton’s use of the unauthorised 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 judgement 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.

“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.”

US Democratic presidential nominee Hillary Clinton leaves after the unscheduled news conference.(Photo: Reuters)

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

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.”

– Reuters