The FBI director's decision to relaunch an investigation into Hillary Clinton's emails has jolted the US political establishment in the final countdown to Election Day on 8 November.
The long-running controversy has poisoned the Democrat's bid for the White House, with opponents accusing Clinton of lying to the 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 on 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.
According to the 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.
Comey has effectively relaunched the probe into Clinton's unauthorised use of a private server, which was closed without any charges being 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 5 July, Comey recommended that Clinton not face criminal charges.
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 the Republicans at the time for declining to recommend charges.
Now he is under attack from the Democrats over the timing of the new revelations, which they say could interfere with the election result.
Defiant Clinton camp doubles down
Initially caught off guard, Clinton's campaign has swung between defiance, counter-attack and painting Trump as a mortal danger since the FBI director revived her email scandal.
The decision by Comey initially dumbfounded Clinton and her allies, throwing what was already America's ugliest presidential election into spasms of shock.
An aide said the former secretary of state, first lady and senator took it "like a champ" but the campaign is furious about the renewed focus on her use of a private email server and frustrated that Comey has still not provided answers to them — or the public.
An angry campaign chairman John Podesta went on the attack, the most senior Democrat in the Senate said Comey may have broken the law and campaign manager Robby Mook decried what he called a "double standard."
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 (email@example.com) 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 2 March, 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.
Why didn't Abedin know?
The email discovery raises an immediate question: How could close aide Abedin have been unaware of their existence?
It's possible that Abedin did not know about the emails on Weiner's computer, forgot about them or for some other reason did not turn them over.
In a sworn deposition taken in June as part of a lawsuit filed by the conservative legal group Judicial Watch, Abedin was asked about what devices she had used to send or receive messages from her account on the clintonemail.com server. As part of the process in 2015 of returning her work-related emails to the State Department, Abedin said she "looked for all the devices that may have any of my State Department" work and provided two laptops and a Blackberry to her lawyers for review.
Abedin made no mention of there being additional devices where her emails might have been saved.
If the FBI finds emails, Abedin sent or received through the clintonemail.com server archived on the device recently recovered from her home, that would appear to conflict with what she told the FBI earlier this year.
In an April interview, Abedin told FBI agents that after she left the State Department in 2013, Clinton's staff transitioned to a different email server and she "lost most of her old emails as a result." She said she had only accessed her clintonemail.com account through a web portal and that she "did not have a method for archiving her old emails prior to the transition."
FBI email action 'may have broken law'
The US Senate's top Democrat blasted FBI chief James Comey for announcing a new review of Clinton's emails, an action he says "may have broken the law."
"As soon as you came into possession of the slightest innuendo related to secretary Clinton, you rushed to publicise it in the most negative light possible," Senate Minority Leader Harry Reid said in a statement.
"Through your partisan actions, you may have broken the law," Reid said, alleging that Comey had violated the Hatch Act, which bars the FBI from influencing elections.
Clinton has demanded the FBI director explain in detail why he had effectively reopened an inquiry declared complete in July, branding Comey's move "deeply troubling" so close to Election Day.
Can this sway the election?
Republican nominee Donald Trump currently lags behind by less than three points 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 percent support to Trump's 39 percent, 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 percent of Americans believe the Clinton email scandal is worse than the Watergate scandal that brought down president Richard Nixon — echoing a favourite Trump talking point.
With inputs from agencies