A new shock has hit Hillary Clinton's presidential campaign, as the FBI has said it's looking into whether there was classified information on a device belonging to the estranged husband of one of her closest aides.
FBI director James Comey said in a letter to Congress on Friday that the bureau had discovered the emails while pursuing an unrelated case.
It's resurfaced an issue that has dogged Clinton for her entire campaign - here's a look at everything we need to know about this latest twist.
So, where did these new emails come from?
The emails referenced in Comey's letter emerged during a wholly separate criminal sexting investigation into former Representative Anthony Weiner. He's the estranged husband of Huma Abedin, one of Clinton's closest aides, a US official said.
Federal authorities in New York and North Carolina are investigating online communications between Weiner and a 15-year-old girl.
It wasn't clear from Comey's letter who sent or received the emails in question or what they were about. And it also wasn't clear what connection - if any - they might have to the earlier Clinton investigation.
It's 10 days before the election. Why is this coming out now?
Well, apparently the emails were found very recently. In his letter to Congress, Comey said he had been briefed only on Thursday by the investigators.
Some have criticised Comey for releasing the letter - and dropping such a significant development - too close to an election. But if he had kept the letter under wraps until after November 8, it would surely have led to criticism that he was sitting on major news until after a new president had been elected.
What kind of political impact might this all have?
The email matter is something Clinton thought was behind her months ago. Her campaign chairman, John Podesta, said it was "extraordinary" for the public to see a letter like that so soon before an election.
And Clinton called on the FBI to immediately release the full details of what it is now examining. She said she didn't expect the additional review to produce conclusions different from the ones reached in July - that was when the investigation was closed without charges.
Republican presidential nominee Donald Trump immediately seized on the news, saying "perhaps finally justice will be done".
But is this disclosure standard for the FBI?
No, but then neither was the Clinton email investigation.
But Comey promised extraordinary transparency as he announced the conclusion of the investigation last July.
"I am going to include more detail about our process than I ordinarily would, because I think the American people deserve those details in a case of intense public interest," he said.
Since then, the FBI has periodically released investigative files - summaries of witnesses who were interviewed - and those materials aren't usually seen by the public.
What's up with Comey's letter being so vague?
The letter's vagueness was immediately seized upon by critics as unacceptable and leaving the American people in the dark.
But the FBI avoids publicly discussing ongoing criminal investigations, or even confirming that it has one open. Plus, it appears from the letter that the FBI is not yet sure what it has.
So, what happens now?
The FBI will review the emails to see if they were classified and were improperly handled.
But it's impossible to say based on Comey's letter that anyone is in greater jeopardy than before.
The FBI announced in July that scores of emails from Mrs Clinton's server contained information that was classified at the time it was sent or received. So, additional emails determined to be classified might do nothing to change the legal risk for anyone who sent them.