Here is a rundown of what we know (and what we don't) so far:
What is the Huma Abedin/Anthony Weiner connection?
The emails were discovered as part of an investigation into former New York congressman Anthony Weiner, who is suspected of having sexually charged communications with a 15-year-old girl. Investigators came across the emails while looking into devices used by Weiner. Weiner is married to — and currently separated from — longtime Clinton aide Huma Abedin, who had access to the same device or devices. The FBI obtained a warrant to begin a review of the emails, an official familiar with the matter said Sunday. The Wall Street Journalreported Sunday that there may be as many as 650,000 emails on the laptop, but it is unclear how many may be relevant to the Clinton email server investigation.
What emails were discovered?
Many things remain vague and unknown about the newly uncovered emails. What we do know is that the FBI believes some "appear to be pertinent" to its investigation into Clinton's use of a private server during her tenure as secretary of State. According to an official close to the investigation, there are thousands of new emails under review. What we don't know is whether they contain any classified material or could have any bearing on the previous investigation.
Could the emails lead to criminal charges against Clinton?
That is impossible to know at this point. In a speech Friday, Clinton said she is "confident" that the new developments won't change the FBI's recommendation not to prosecute her for improperly handling classified materials. Although Comey could not put a timetable on how long it will take to review the new material, a source tells USA TODAY that is unlikely it will be completed before Election Day.
Why did Comey make the review public?
In his letter to lawmakers informing them of the latest developments, Comey said he thought it was important to update them "in light of my previous testimony," which said the investigation had been concluded. A bipartisan group of nearly 100 former federal prosecutors and senior Department of Justice officials, including former Attorney General Eric Holder, have signed a letter saying they were "astonished" and "perplexed" by Comey's decision.
Did Attorney General Loretta Lynch approve of Comey's letter to lawmakers?
No. Lynch objected to Comey's decision to notify Congress that the FBI was reviewing newly discovered email, an official familiar with the matter told USA TODAY. Lynch based her objection on a long-held Justice Department policy that federal authorities should not take any action that may interfere with an election.
Did Lynch try to stop Comey?
Yes. Lynch shared her objections just hours before Comey sent the letter, according to USA TODAY's source. The FBI director weighed the attorney general's advice during a spirited discussion of the matter Thursday and early Friday, but in the end, Comey felt compelled to act.
What is the Clinton camp saying?
Top Clinton campaign officials urged Comey on Sunday to provide more details, charging that his "inappropriate" letter is fueling conspiracy theories that could hurt the Democratic nominee just days before the election. Clinton campaign chairman John Podesta called the letter "long on innuendo, short on facts." Senate Minority Leader Harry Reid sent a letter to Comey on Sunday saying he may have violated a federal law that bars federal officials from using their authority to influence an election.
What is the Trump camp saying?
Friday's announcement has emboldened Trump and aides who say the Nov. 8 election should be about the character of the Democratic nominee. "We have one ultimate check on Hillary’s corruption, and that is the power of the vote," Trump told supporters Sunday at a rally in Las Vegas.
On MSNBC's Morning Joe Monday, Trump's campaign manager, Kellyanne Conway, said the campaign wasn't going to focus on the investigation on the campaign trail.
"We don’t plan on making the Comey investigation the main centerpiece of our messaging. Mr. Trump will continue to talk about what he’s talked about all along," Conway said.
What do the polls say?
Most polls still give Clinton a lead, but Comey's letter has roiled the election in ways that campaigns, pollsters, and analysts are still trying to assess.
When is the election?
Tuesday, November 8.