Hillary Clinton's use of a private email server while secretary of State has dogged her campaign since early last year. Here's how the latest controversy kicked off:
THE WEINER CONNECTION
The latest controversy got rolling when FBI Director James Comey told Congress on Friday that the FBI had found emails that could be relevant to an inquiry into Clinton's use of a private email server while Secretary of State.
The emails were found on a device that belonged to Anthony Weiner, the estranged husband of close Clinton aide Huma Abedin.
Many would recognise Weiner's name as the New York City mayoral candidate who was caught in 2011, 2013 and again this year sending numerous women sexually explicit text messages and photos of himself undressed.
The FBI is involved because federal authorities in New York and North Carolina are investigating online communications between Weiner and a 15-year-old girl.
A person familiar with the investigation, who lacked authority to discuss the matter publicly and insisted on anonymity, said the device that appears to be at the centre of the new review was a computer that belonged only to Weiner and was not one he shared with Abedin.
As a result, it was not a device searched for work-related emails at the time of the initial investigation. The person said it is "news to (Abedin)" that her emails would be on a computer belonging to her husband.
Abedin announced her intention to separate from Weiner earlier this year.
She told lawyers in June in a deposition that, like millions of internet users who don't manage their inboxes, she never deleted old emails on her devices, either at work with Ms Clinton or at home with Mr Weiner.
"I didn't have a practice of managing my mailbox other than leaving what was in there sitting in there," Abedin said.
"I didn't go into my emails and delete State.gov emails. They just lived on my computer. That was my practice for all my email accounts. I didn't have a particular form of organising them. I had a few folders, but they were not deleted. They all stayed in whatever device I was using at the time or whatever desktop I was on at the time."
In February 2013, Abedin signed a routine State Department document under penalty of perjury in which she promised to "turn over all classified or administratively controlled documents and materials" before she left her government job, and promised that she was not retaining copies, "including any diaries, memorandums of conversation or other documents of a personal nature."
It was not immediately clear what the emails were about or what significance, if any, they carried to the email investigation.
In a letter sent to members of Congress on Friday, Comey said the FBI had recently come upon new emails while pursuing an unrelated case.
The FBI is investigating whether there is classified information in the new emails uncovered during the sexting investigation of Weiner.
Comey said in his letter that the emails prompted investigators to take another look at whether classified information had been mishandled, which had been the focus of its recently closed, criminal probe into Clinton's use of a private email server.
Comey couldn't guarantee that the latest focus of the investigation would be finished before Election Day.
As Democrat Senator Dianne Feinstein of California noted, the decision to reveal the existence of the emails was particularly troubling because so many questions were unanswered.
"It's unclear whether these emails have already been reviewed or if Secretary Clinton sent or received them," she said.
"In fact, we don't even know if the FBI has these emails in its possession.
"Without knowing how many emails are involved, who wrote them, when they were written or their subject matter, it's impossible to make any informed judgment on this development."
She said Comey's announcement played right into the political campaign of Donald Trump, who was using the letter for political purposes.
"The FBI has a history of extreme caution near Election Day so as not to influence the results, Dianne Feinstein said in a statement. "Today's break from that tradition is appalling."
Clinton said Friday that "the American people deserve to get the full and complete facts immediately. She urged the FBI to "explain this issue in question, whatever it is, without any delay." "Let's get it out," she said.
She also noted: "The director himself has said he doesn't know whether the emails referenced in his letter are significant or not. I'm confident whatever they are will not change the conclusion reached in July."
COULD THEY BRING DOWN CLINTON?
Ms Clinton's use of a private email server while secretary of State has dogged her campaign since early last year.
A year-long FBI investigation concluded in July and focused on whether Clinton sent or received classified information using a server in the basement of her New York home, which was not authorised to handle such messages.
Abedin, who was Clinton's deputy chief of staff at the State Department and remains a key player in her presidential campaign, was interviewed by the FBI as part of its investigation.
Comey said in July that his agents didn't find evidence to support a criminal prosecution or direct evidence that Ms Clinton's private server was hacked.
The FBI investigation concluded without charges but Comey did criticise the Democrat and her aides for being careless with classified material.
Early in the month, WikiLeaks began releasing thousands of hacked emails of Hillary Clinton's campaign chairman, John Podesta. The emails exposed campaign infighting and raised ethical questions about her family's charitable foundation.
The emails would have driven news coverage entirely if not for the Access Hollywood recording in which Donald Trump bragged to TV host Billy Bush about sexually assaulting women. Trump dismissed his comments as "locker room talk," but several women have come forward to say Trump made unwanted sexual advances to them in the past.
The timing of Comey's letter just 11 days before the election drew criticism from Democrats and some Republicans who cast it as unprecedented and potentially tipping the scales in the presidential race in favour of Republican Donald Trump.
Energised by the news, the GOP presidential nominee has rallied his supporters, calling the latest developments worse than Watergate and arguing that his candidacy has the momentum in the final days of the race.
"We never thought we were going to say 'thank you' to Anthony Weiner," Mr Trump said in Nevada.
Mr Trump also highlighted reports that the Justice Department had discouraged the FBI from alerting Congress to the unexpected discovery of the emails, and said the department is trying "so hard" to protect Ms Clinton.
In an apparent departure from the wishes of top Justice Department leaders, FBI director James Comey acted independently when he sent several members of Congress a letter about the emails on Friday, according to the official, who was not authorised to discuss internal deliberations and spoke on condition of anonymity.
Matthew Miller, a former chief spokesman for the Justice Department, was dismayed by the timing of Comey's letter.
"Longstanding DOJ and FBI practice is you don't say anything publicly close to an election that can possibly influence that election," Mr Miller said.
Clinton described the timing of the move as "unprecedented" and "deeply troubling".
FBI investigators in the Weiner sexting probe knew for weeks about the existence of newly discovered emails potentially related to the investigation Clinton's private email server, a law enforcement official said Sunday.
Comey, who has talked often about the FBI's need to be accountable to the public, promised extraordinary transparency about the investigation and during intervening months has authorised the release of investigative files from the case, which are normally kept confidential.
In an internal email to FBI employees, Comey wrote: "Of course, we don't ordinarily tell Congress about ongoing investigations, but here I feel an obligation to do so given that I testified repeatedly in recent months that our investigation was completed. I also think it would be misleading to the American people were we not to supplement the record." The Associated Press acquired the email Friday night.
Trump campaign manager Kellyanne Conway said Comey was in "an impossible spot" when he acknowledged the FBI was looking into the messages.
"Had he sat on the information, one can argue that he also would be interfering in the election," by failing to disclose the review, Conway said.
Clinton's advisers and fellow Democrats pressured Comey anew to release more details about the emails, including whether he had even reviewed them himself.
Tim Kaine, Clinton's running mate, said Comey owed it to the public to be more forthcoming about the emails under review by the FBI with only nine days remaining before the November 8 election.
Calling Comey's announcement "extremely puzzling," Kaine said that if Comey "hasn't seen the emails, I mean they need to make that completely plain."
Clinton's campaign chairman, John Podesta, said Comey's handling of the matter was "inappropriate."
Late Saturday, four senior Democratic senators urged the Justice Department and the FBI to provide more detailed information by Monday about what investigative steps are being taken, the number of emails involved and what is being done to determine how many of the emails are duplicative of those already reviewed by the FBI.