Comey had dropped a bombshell 11 days from the election when he informed Congress that the FBI had discovered emails in its separate investigation of Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin, that could be connected to its investigation of whether Clinton mishandled classified information by using a private email server.
It's impossible to know before results are tallied what impact Comey's actions -- first raising a vaguely worded red flag 11 days out, and then lowering it two days from the election -- will have on the contest. But the news could help Clinton put to rest a controversy that has dogged her in the 2016 race's closing days, helping Donald Trump narrow a polling gap nationally and in key battleground states.
"Based on our review, we have not changed our conclusions that we expressed in July," Comey said in the letter to top Republicans on the House Oversight Committee.
Comey's letter was the culmination of a fast-paced review of the newly discovered email, law enforcement sources said Sunday.
"We went through this as fast as we could," a senior law enforcement official told CNN.
The emails, thousands found, were mostly duplicates of what had already been seen and personal emails, law enforcement officials said in explaining how the conclusion was reached so quickly. The laptop which was found was around a decade old, with lots of personal content on it not relevant to the investigation, according to one source.
Comey sent the letter as soon as possible, the senior law enforcement official said. He was "very careful" about how the letter was crafted given the sensitivities.
Attorney General Loretta Lynch was informed ahead of time that the FBI director was sending the updated letter to congress informing them of that there would be no change in the FBI stance on Hillary Clinton, a Justice official told CNN on Sunday.
Lynch supported the decision.
"Everyone is on the same page," the source said.
Since there is no change to the original findings, the Justice Department has no need to weigh in further, the official said.
"The Department of Justice and the FBI dedicated all necessary resources to conduct this review expeditiously," the Justice Department said in a brief statement released Sunday afternoon.
Campaigns weigh in
Clinton spokesman Brian Fallon said her campaign is validated in its belief that nothing would change.
"We were always confident nothing would cause the July decision to be revisited. Now Director Comey has confirmed it," tweeted Clinton spokesman Brian Fallon.
Donald Trump's campaign manager, Kellyanne Conway, told MSNBC Sunday after the news broke: "He's mishandled this investigation from the beginning, and this proves it yet again."
Trump himself did not directly address the FBI's latest announcement as he took the stage at a rally in Minneapolis more than 20 minutes after news of Comey's letter to Congress surfaced, but continued to argue that Clinton "will be under investigation for a long, long time."
But the Republican nominee, who has expressed hope in the last week that the FBI's review of new emails might yield an indictment, emphasized to his supporters on Sunday that Clinton is "protected" by powerful forces.
"Well, you have to understand it's a rigged system and she's protected," Trump said, though he offered no evidence to back up his theory.
'It opened a wound'
A senior Democrat close to Clinton's campaign told CNN that "it's impossible to fully undo the damage of the last nine days."
Internal campaign polling, the source said, found that some independents and Republican women fled Clinton after the original Comey announcement, robbing the Democratic nominee of a constituency that she'd hoped would turn her contest with Trump into a blowout.
"It opened a wound that cannot be quickly healed," the Democrat said.
However, the Democrat called it a relief that the "matter is resolved" and that Clinton's campaign plans to "make everyone know about" Comey's Sunday letter.
Comey's last-minute announcement gives Clinton an opportunity for an I-told-you-so moment -- but it's unlikely to undo the political damage of his initial announcement.
"We have seen Director Comey's latest letter to the Hill. We are glad to see that he has found, as we were confident that he would, that he had confirmed the conclusions he reached in July and we are glad that this matter is resolved," said Jennifer Palmieri, Clinton's communications director, speaking with reporters after the news broke.
Trump and his allies have seized on that announcement, using it claim Clinton is likely to face criminal charges.
"If she were to win, it would create an unprecedented constitutional crisis," Trump claimed Saturday night in Reno, Nevada. "In that situation we could very well have a sitting president under felony indictment and ultimately a criminal trial. It would grind government to a halt."
The political benefit for Trump has been that Republicans who'd been skeptical of their party's nominee have largely followed vice presidential nominee Mike Pence's calls to "come home" to the party -- finding Trump less objectionable than Clinton.
It's not just Trump's campaign that seized on Comey's actions. Down-ballot Republicans -- particularly Senate candidates who had been forced to answer for Trump's statements for months -- used the moment to change the topic, shifting to the more comfortable ground of attacking Clinton.
The last-minute Sunday move from Comey could also provide new fuel for Trump's claims of a "rigged" system -- allowing him to cast the FBI director's intervention in the presidential election in a new light.
Comey and the FBI went from Trump's scapegoat to the Republican nominee's hero, with Trump claiming Comey had redeemed himself just in time.
Newt Gingrich, a top Trump ally, tweeted "Comey must be under enormous political pressure to cave like this."
Dems, GOP react
House Speaker Paul Ryan dismissed the significance of Comey's latest letter.
"Regardless of this decision, the undisputed finding of the FBI's investigation is that Secretary Clinton put our nation's secrets at risk and in doing so compromised our national security," Ryan said in a statement. "Fortunately, the American people have the opportunity to ensure Secretary Clinton never gets her hands on classified information again."
Capitol Hill Democrats praised Comey's decision to release more information.
"While the original letter should never have been sent so close to an election, the expeditious review of these emails should put to rest -- once and for all -- the irresponsible speculation indulged in by the Trump campaign and others," Rep. Adam Schiff, the top Democrat on the House Intelligence Committee said. "Voters can now make their decision based on the merits, and that decision should be simple: it is the choice between a woman superbly qualified to be commander in chief, and a man patently unfit for office."
Republican National Committee Chairman Reince Priebus issued a statement saying, "None of this changes the fact that the FBI continues to investigate the Clinton Foundation for corruption involving her tenure as secretary of state. Hillary Clinton should never be president."