The long-running controversy has poisoned the Democrat's bid for the White House, with opponents accusing her of lying to Congress and putting state secrets at risk.
Here's what we know about the case:
New email trove
In a bombshell announcement made on Friday, 11 days before Election Day, FBI Director James Comey revealed that the agency was reviewing a new batch of emails that "appear to be pertinent to the investigation" into Clinton's use of a private email server as secretary of state.
US networks reported Sunday that the FBI had obtained a warrant to start combing through the emails -- about which Comey has provided no details -- to see if they contain classified information.
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According to US media, the new emails were discovered on the computer of disgraced congressman Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin.
Weiner, who resigned in 2011 after sending explicit online messages, is under federal investigation over allegations he sent sexual overtures to a 15-year-old girl.
Probe reopened?
The FBI's Comey has effectively relaunched the probe into Clinton's unauthorised use of a private server, which was closed without any charges filed in July.
After investigating emails sent and received by the former secretary of state, the FBI had concluded that 110 messages contained classified information at the time they were sent.
More than 2,000 emails were later categorised as classified or containing confidential or secret information, fueling attacks by Clinton critics who said she put national security at risk.
On July 5, Comey recommended that Clinton not face criminal charges.
Hillary Clinton's dilemma (AFP) Source: AFP
But he criticised her as "extremely careless" in the handling of "very sensitive, highly classified information" and said she potentially exposed classified information to hostile foreign governments.
A Republican appointee, Comey came under fire from Republicans at the time for declining to recommend charges.
Now he is under attack from Democrats over the timing of the new revelations, which they say could interfere with the election result.
How it all began
Clinton contravened official guidelines during her tenure as secretary of state from 2009 to 2013 by using a personal email address () via a private server installed in her Chappaqua, New York home instead of a government account, potentially exposing confidential information to hackers.
The Democratic presidential candidate apologised for what she called a mistake, but maintains she did nothing illegal.
The matter was made public in a New York Times article published on March 2, 2015.
Clinton's personal account accumulated more than 60,000 emails over four years. At the State Department's request, she turned over around half of them, pertaining to her official duties, in October 2014, and deleted those deemed personal.
The State Department published the emails in waves on a judge's orders.
Can this sway the election?
Republican nominee Donald Trump currently lags less than three points behind Clinton in the national poll average following a tightening of the race which had begun even before the FBI's announcement.
A Politico poll conducted over the weekend has Clinton leading with 42 per cent support to Trump's 39 per cent, in a four-way race with two lesser known candidates.
It remains unclear how much the Comey announcement will move the needle on a race where Trump continues to lag behind Clinton in key battleground states.
But for Republicans, whose champion has been damaged by a string of allegations of sexual misconduct, the renewed focus on Clinton's emails is a welcome boost.
There is so far no indication whether the FBI could eventually revisit its decision not to charge Clinton, but Republicans have seized on the controversy to brand Clinton as criminally reckless.
"We would have a criminal trial for a sitting president," Trump told supporters in Michigan.
The Politico poll found that 45 per cent of Americans believe the Clinton email scandal is worse than the Watergate scandal that brought down president Richard Nixon -- echoing a favorite Trump talking point.
