Hillary Clinton on Friday called on the FBI to release more information about its review of emails that may be related to its investigation into her private server.
The FBI has revealed it is looking closely at Ms Clinton once more after a series of new emails sent from the Democratic presidential nominee via a private server came to light.
The bureau's director, James Comey, released a letter on Friday (US time) saying the emails, which were being looked at in relation to an unrelated case, appeared to be "pertinent to the investigation".
That case is believed to be in relation to Anthony Weiner, a disgraced former Congressman who is suspected to have sent messages of a sexual nature to a 15-year-old girl earlier this year.
The probe into those allegations saw both Mr Weiner and his wife Huma Abedin's personal devices seized by the FBI. Ms Abedin is one of Ms Clinton's top aides, and holds a significant role in her presidential campaign.
While Mr Weiner could be handed a sentence of up to 30 years' jail if found guilty of the offence, Ms Clinton could also find herself in serious trouble over the new batch of emails, which a federal official says number in the thousands.
In July this year, the FBI's investigation into Ms Clinton's use of a personal email server had ended with a recommendation of no criminal charges - but Mr Comey had described Ms Clinton as "extremely careless" for committing such an error.
Mr Comey has now published a letter saying the new emails have given the FBI enough reason to reopen the case.
"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 said.
"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 this additional work, I believe it is important to update your committees about our efforts in light of my previous testimony."
The discovery of the new set of emails may throw a serious spanner in the works for Ms Clinton, just 11 days ahead of the presidential election.
She has charged well ahead of Republican Donald Trump in the race for the White House, according to a number of polls - but the latest revelations are a hit to her efforts to win a number of swing states.
The Democratic presidential candidate says the public deserve to have as much information as possible before the November 8 election date.
She told a brief news conference she did not know what the messages in question contain.
"Right now, your guess is as good as mine, and I don't think that's good enough," she said.
Ms Clinton said she's confident investigators won't find information that would cause the FBI to change its decision to close the investigation without filing charges in July.
Ms Clinton spoke hours after the FBI announced it was reviewing newly discovered emails to see if they are relevant to its closed investigation into her private email server.
Ms Clinton's running mate Tim Kaine said it was "very, very troubling" that the FBI was releasing information about a new probe into emails that may relate to Clinton just 11 days before the election.
The Democratic vice presidential nominee was commenting on the development in an interview with Vice News.
Mr Kaine added the FBI director needed to provide more details on the situation. He suggested it was troubling that members of the press were finding out information before campaign officials.
Predictably, Mr Trump seized on the news, saying "Hillary Clinton's corruption is on a scale we have never seen before" and asserting that the investigation was "bigger than Watergate".
"I have great respect for the fact that the FBI and Department of Justice are now willing to have the courage to right the horrible mistake that they made," he told crowds at a rally in New Hampshire, who at one point repeatedly chanted the phrase 'lock her up'.
"This was a grave miscarriage of justice that the American people fully understood and is about to be corrected."
Speaker of the US House of Representatives Paul Ryan said Ms Clinton "only has herself to blame" for the investigation being reopened.
"She was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information," he wrote on Twitter.
"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."
Meanwhile, the Republican chairman of the Senate Judiciary Committee is asking for a briefing from the FBI.
Iowa Senator Charles Grassley said in a statement that the FBI's letter to congress that it is looking for classified information in newly discovered emails that appear to be related to its probe of Ms Clinton's email practices "was unsolicited and, quite honestly, surprising". He said it left more questions than answers.
"Congress and the public deserve more context to properly assess what evidence the FBI has discovered and what it plans to do with it," Mr Grassley said.
Mr Grassley said at a campaign event in Iowa on Friday that he thinks the revelation is "going to be a real problem for the Clintons" but he has to wait for more information.
Newshub. / AP