"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."
Clinton called on Comey to swiftly release more information, saying he must "explain everything right away, put it all right on the table."
Clinton also swiped at Republican rival Donald Trump, saying he's "doing his best to confuse, mislead and discourage the American people" over the issue.
"Of course, Donald Trump is already making up lies about this," Clinton said.
The Democratic nominee's comments here marked an escalation in the Clinton campaign's full-out war against Comey in the final 10 days of the election.
Earlier in the day, just hours after Comey made the stunning announcement on Friday that the FBI is examining newly unveiled emails that appear to be "pertinent" to the now-closed investigation into Clinton's private server, top campaign officials unleashed a blistering attack on the FBI director, accusing him of being irresponsibly "light on facts" and "heavy on innuendo."
On a conference call with reporters, Clinton campaign chairman John Podesta accused Comey of failing to be "forthcoming with the facts." Podesta blasted both the timing and contents of the letter Comey sent to congressional leaders on Friday, summing the director's actions as "providing selective information."
Campaign manager Robby Mook echoed Podesta's call for more information from Comey and suggested that the FBI has now waded into political territory.
"The Justice Department's longstanding practice is: Don't do anything seen as trying to influence an election," Mook said. "It's completely unfair to Secretary Clinton and it's really unfair to the voters."
'Hillary's not going to be distracted'
The sharp criticism marked an extraordinary rebuke of the head of an agency historically known for its political independence. And it guarantees that Comey -- already a polarizing presence for his role in probing Clinton's email use as secretary of state -- will be a central political figure in the election's final days.
As Clinton prepared to fly to Florida for a two-day swing through the battleground state, Podesta was defiant: "We're not going to be distracted and Hillary's not going to be distracted in the final days of this election over nothing."
Both Podesta and Mook emphasized the possibility that the emails that the FBI is now examining -- uncovered as part of the bureau's investigation into Clinton aide Huma Abedin's estranged husband, Anthony Weiner -- may not reveal new information.
"Reports indicate that many of these emails are likely to be duplicates," Podesta said. "It's in fact entirely possible all the emails in question are just that -- duplicates."
Donald Trump continued to trumpet the news at a campaign stop in Golden, Colorado, Saturday afternoon, wondering if Clinton would keep Abedin as an adviser.
"Huma's been a problem," Trump said. "I wonder if Huma's going to stay there. I hope they haven't given Huma immunity because it seemed that everybody that walked down the sidewalk got immunity. She knows the real story. She knows what's going on."
Abedin, one of Clinton's closest confidantes, regularly travels with Clinton and was with the candidate on Friday when the FBI news broke. However, Abedin was notably absent on Saturday as Clinton traveled to Florida.
Comey was sworn in as FBI director in 2013 for a 10-year term. That means if Clinton were to win in November, Comey would serve under her administration unless she chose to remove him.
A Clinton aide later said Saturday that Clinton took the FBI news "like a champ." The aide added that Clinton and her top aides were well aware that the last two weeks of the campaign would not be entirely smooth and that Clinton's reaction reflected that.
"She is in a good state of mind," the aide added.
The new attacks on Comey are a sudden turn for Clinton's campaign, which sang the FBI director's praises for his initial findings in the Clinton email case.
Clinton aides, in conversations with reporters and in statements, highlighted the fact that Comey was a Republican known for his fierce independence, touting him -- and his investigators -- as career professionals. Surrogates were told to highlight Comey's testimony to Congress and note his political affiliation and track record.
In an interview on CNN's "Situation Room" in July, Clinton press secretary Brian Fallon went after Republicans for criticizing Comey after the FBI concluded it would not press criminal charges against Clinton.
"It seems they were disappointed with the outcome of the FBI investigation so they decided to put the director in a hot seat and second-guess his decision," Fallon said at the time. "I think it is a bad look for House Republicans to be second-guessing a career prosecutor who is a registered Republican, No. 2 official at the Justice Department under George Bush and was even deputy council on the committee investigating the Clintons in the 1990s."
Ironically, the Clinton campaign now finds itself voicing similar concerns as some Republicans when it comes to the FBI's handling of emails uncovered in its Weiner investigation.
Former House Speaker Newt Gingrich, an outspoken Trump supporter, wrote on Twitter Saturday: "If there are really more than 10,000 emails in the weiner-Abedin computer the FBI should release all Monday. Americans have a right to know," he wrote.
Gingrich added: "We should not be forced to vote with ten thousand or more emails still hidden by the FBI. John Podesta and Hillary Clinton are right."