Eleven days before the 2016 presidential election, Comey announced that the FBI had discovered additional emails and is reviewing them to see whether they are related to the bureau's investigation into Clinton's handling of classified information.
Here's what you need to know -- and how it might affect Clinton, Donald Trump and the presidential race:
It started with the FBI's investigation into Anthony Weiner, the disgraced former New York congressman who was caught over the summer exchanging lewd and sexually suggestive messages with a 15-year-old girl.
Weiner's estranged wife is Clinton adviser Huma Abedin
. And in their look into Weiner's sexting allegations, which began on September 22, investigators from the FBI's New York field office discovered Abedin's emails on Weiner's laptop -- with initial data showing those emails went through Clinton's server.
It was enough to lead FBI Director James Comey
to conclude the emails would need to be reviewed to see if he'd need to reopen the investigation he'd closed in July on whether Clinton kept classified information on the private email server she used during her tenure as secretary of state.
Comey was made aware of the emails' existence by mid-October, law enforcement sources have said. He was given a full briefing on Thursday.
Comey decided Friday after a series of "long grueling meetings" with top FBI executives that the FBI needed to review to see whether the emails were related to its investigation into Clinton's server, and a letter would be sent to Congress about the development, a law enforcement source told CNN.
And so, on Friday -- 11 days from the election -- Comey informed eight Senate and House chairmen, who are Republicans, and copied the ranking Democrats on their panels.
Does Comey's decision break with precedent?
Everything about this is beyond precedent.
There's never been a major party nominee under FBI investigation leading up to the election before. And how it's being handled is equally out of the ordinary.
Comey gave advance notice to top officials at the Justice Department before sending his letter to lawmakers Friday -- but he didn't get sign-off from those officials, and wasn't seeking it. Instead he made an independent decision to go against longstanding Justice Department and FBI practice to not comment publicly about politically sensitive investigations within 60 days of an election, a law enforcement official said.
Comey's decision adds to the unusual role he has played in the Clinton email probe, which some critics have said usurped the role of prosecutors in the Justice Department whose job is to review FBI findings and make decisions on whether to bring charges.
Comey made the unprecedented move in July to publicly announce that he would recommend no charges be brought in the email investigation, saying that no reasonable prosecutor would bring such charges. He made the July announcement without providing prior notice to Justice officials.
That was in part because of another extraordinary part of the Clinton email investigation: A tarmac meeting between Bill Clinton and Lynch -- who, because of that meeting, left decisions on whether to prosecute Clinton to others because of the conflict.
How mad are Democrats about Comey's Friday move?
Senate Minority Leader Harry Reid said in a letter to Comey on Sunday that he may have violated the Hatch Act, barring political activity by federal officials.
"I am writing to inform you that my office has determined that these actions may violate the Hatch Act," Reid said in his letter to Comey. "Through your partisan actions, you may have broken the law."
White House Press Secretary Josh Earnest said Monday that President Barack Obama does not believe Comey is attempting to influence
"The President doesn't believe that he's secretly strategizing to benefit one candidate or one political party," Earnest said. "He's in a tough spot, and he's the one who will be in a position to defend his actions in the face of significant criticism from a variety of legal experts, including individuals who served in senior Department of Justice positions in administrations led by presidents in both parties."
Clinton's campaign is focused largely on the reality that Comey's actions are out of step for the Justice Department and the FBI -- but now that Comey has made his move, the political consequences can't be undone.
On Monday, former Attorney General Eric Holder wrote an op-ed in the Washington Post criticizing Comey's decision
, titled: "James Comey is a good man, but he made a serious mistake."
"I served with Jim Comey and I know him well. This is a very difficult piece for me to write. He is a man of integrity and honor. I respect him. But good men make mistakes. In this instance, he has committed a serious error with potentially severe implications," Holder wrote.
Calling Comey's letter "a stunning breach of protocol," Holder said that the departure from policy broke "fundamental principles. I fear (Comey) has unintentionally and negatively affected public trust in both the Justice Department and the FBI. And he has allowed — again without improper motive — misinformation to be spread by partisans with less pure intentions."
What does this have to do with Hillary Clinton?
It's too soon to know.
The FBI discovered emails sent to or from Abedin -- one of Clinton's longest-serving and most-trusted aides.
But it's not clear whether those are the same emails the FBI already reviewed in its investigation into Clinton's private email server that concluded in July with an admonition from Comey -- but no recommendation of charges.
There could be additional emails that Clinton didn't turn over to the State Department and the FBI didn't discover during its investigation. Even if any such emails exist, it still might not mean much because they could be personal in nature.
Comey wrote to Congress that "the FBI cannot yet assess whether or not this material may be significant."
What will happen next?
The Justice Department obtained a warrant Sunday that will allow it to begin searching the emails found on Weiner's laptop to determine if they contained any classified information.
However, in his letter to Congress, Comey all but guaranteed the FBI wouldn't be done with its review prior to Election Day. He said he "cannot predict how long it will take us to complete this additional work" -- and since the initial investigation took months, completing even a smaller version in less than nine days is improbable.
A lot hinges on how many of the emails are new. If the FBI has gone through most of them before, it could speed the process. But if some are new, the FBI could need to consult with other agencies regarding the sensitivity of the information.
That means the short-term political problem for Clinton is uncertainty in the days before the election.
At this point, it appears Abedin herself doesn't know what investigators will discover.
"The possibility that this device contains any emails of hers is news to her," a source familiar with the investigation and civil litigation told CNN. "The device supposedly at issue now belonged to Anthony, not her."
How will this impact the election?
It'll take a couple days before new polls will show the full impact of the latest revelation.
But for Clinton, what's troubling is that the development gives new life to a story that has fueled voters' sense that she's untrustworthy -- perhaps her biggest political liability -- at exactly the wrong time, as her campaign's early voting operation is at full stride and with little time left before November 8.
It's clear Clinton's campaign senses the danger. Her top aides are hardly hiding their frustration -- and they're directing that frustration at Comey.
"He might have taken the first step of actually having looked at them (the emails) before he did this in the middle of a presidential campaign, so close to the voting," Clinton campaign chairman John Podesta told CNN's Jake Tapper on "State of the Union."
Clinton herself weighed in Saturday, questioning Comey's decision to act so close to the election -- and without more information to explain just what the bombshell he dropped actually is.
"It's pretty strange to put something like that out with such little information right before an election," Clinton told supporters in Daytona Beach, Florida, where the crowd booed at the mention of Comey's letter. "In fact, it's not just strange, it's unprecedented and it's deeply troubling because voters deserve to get full and complete facts."
Trump, meanwhile, is reveling in Clinton's political trouble -- with his surrogates tamping down their charges of politicization within the Justice Department, and Trump even cracking a joke on Sunday.
"We never thought we were going to say thank you to Anthony Weiner," Trump said at a rally in Las Vegas.