Just days before the US presidential election and with Americans in many states already going to the polls for early voting, the Federal Bureau of Investigation (FBI) made a stunning announcement that it had discovered new e-mails that might be relevant to a completed investigation of Mrs Hillary Clinton's private server.
The announcement, made in a vague 166-word statement to Congress, has left many voters puzzled over what to make of a case involving national security secrets, a disgraced congressman, racy text messages and a dispute among the country's top law enforcement officers.
Here's what we know so far:
What was in the FBI announcement?
FBI director James Comey sent a letter to Congress on Friday (Oct 28) that said its agents had uncovered new e-mails that may be connected to the investigation on Mrs Clinton's use of private server when she was secretary of state. That investigation had examined whether Mrs Clinton and her aides had mishandled classified information by sending it through her private e-mail server. The inquiry was completed in July with no charges filed.
Mr Comey said agents would review the new e-mails to see whether they contained classified information. The letter was sent 11 days before the presidential election on Nov 8 and it set off fierce criticism of Mr Comey for appearing to meddle in politics.
The FBI director's letter did not reopen the inquiry, though some Republicans, including Republican presidential candidate Donald Trump, characterised the move that way.
Agents could open a new inquiry if they find evidence that the earlier investigation had been impeded or that classified materials had been intentionally mishandled.
Where did these new e-mails come from?
The FBI director did not say in his letter. But law enforcement officials briefed on the investigation said agents had discovered the e-mails on a laptop owned by Mr Anthony Weiner, a disgraced former congressman and estranged husband of Mrs Clinton's top aide Huma Abedin.
Last month, the FBI began investigating allegations that Mr Weiner had exchanged sexually explicit messages with a teenager. On Oct 3, agents in New York executed a search warrant to obtain the laptop, his iPhone and iPad. While searching the laptop, they were said to have found evidence of a trove of e-mails similar to the ones that had been examined in the Clinton investigation.
Mr Comey decided that agents should examine those newly discovered e-mails to determine whether they contained national security information.
Why does the FBI care if there is classified information in the e-mails?
Under federal law, mishandling national security information is a crime, one that the FBI is responsible for investigating.
In 2015, the bureau began investigating the personal e-mail account that Mrs Clinton had used exclusively as secretary of state. As part of that investigation, the bureau tried to find every electronic device - phones, tablets, computers - that Mrs Clinton and her aides used. Agents could not find many of them, including several of Mrs Clinton's mobile phones and two iPads.
The agents knew that those devices, and others they were not aware of, might someday surface. But they completed the Clinton case because they found no evidence that anyone had intentionally broken the law.
The newly discovered e-mails may - or may not - provide new information to the FBI.
Why did Mr Comey send the letter?
In July, Mr Comey told Congress that the Clinton investigation was complete but if new information came to light, the bureau would examine it. He pledged to be as transparent as he could with Congress about the investigation, and has since made public hundreds of pages of documents related to the inquiry.
According to senior FBI officials, Mr Comey felt that he would be breaking his pledge of transparency to Congress if he did not reveal the new information from the Weiner case. And he believed that the bureau would be accused of suppressing details to benefit Mrs Clinton - an accusation he believed could do lasting damage to the FBI's credibility.
What does all of this mean for Mrs Clinton and her campaign?
The impact is not yet clear. What is evident is that a campaign that has largely been a referendum on Mr Trump - particularly since the first presidential debate - is now not so clear-cut. The e-mail development will certainly matter, but the question is just how much.
Twenty million people have voted, and millions more have determined who they will support. The country was politically polarised before this election, and opinions are overwhelmingly cemented about these two household-name nominees.
The new development could matter most in down-ballot races. After being on the defensive for weeks because of Mr Trump's behaviour, Republican candidates now have a more helpful news media environment in which to make their closing arguments. And Republican voters who are otherwise demoralised may have been given one final nudge to show up on polling day.
What happens now?
In the coming days, the FBI will begin conducting a smaller version of the larger investigation it completed in July. Agents will go through the e-mails found on the laptop to determine whether they contain classified information. If so, the bureau will again look at the question of whether anyone intentionally committed a crime.
Clinton campaign officials have said Ms Abedin gave the authorities all the electronic devices she believed had work-related e-mails on them. Many of the newly discovered messages are likely to be duplicates of others the FBI has already examined, investigators said. The review will be conducted by the same Washington-based FBI agents who led the investigation into Mrs Clinton's e-mails.
FBI agents are all but certain that it will not be completed by Election Day, and believe it will take at least several weeks.
Is anyone in legal jeopardy?
The FBI did not recommend charges in July because agents had found no evidence that anyone had tried to impede their investigation or intentionally mishandle classified materials. If the new e-mails indicate that type of behaviour, the FBI will most likely want to investigate further.
How rare is it for the FBI to make a development like this public?
Extremely rare. At times during trials or after cases are closed, the FBI finds new evidence and either discloses it to defence lawyers or reopens a case. An FBI director has never made such a disclosure to Congress so close to a presidential election.
More US election stories online at str.sg/election2016.