Hillary Clinton lashed out at the FBI's handling of a new email review, leading a chorus of Democratic leaders who declared the bureau's actions just days before the election "unprecedented" and "deeply troubling".
Emboldened Republican rival Donald Trump seized on the reignited email controversy, hoping to raise new doubts about Mrs Clinton's trustworthiness.
Rallying supporters in Florida, Mrs Clinton pressed FBI Director James Comey to put out the "full and complete facts" about the review into a cache of recently discovered emails.
Clinton backers criticised Mr Comey's letter to Congress about the new emails as severely lacking crucial details.
"It is pretty strange to put something like that out with such little information right before an election," Mrs Clinton said, telling Mr Comey: "Put it all out on the table."
She accused Mr Trump of using the issue to confuse and mislead voters in the final leg of the campaign for the November 8 election.
Mrs Clinton appeared at an outdoor concert in Miami with singers Jennifer Lopez and Marc Anthony, and she urged voters not to let Mr Trump get away with "stoking fear" and "insulting one group of Americans after another".
The controversy over Mrs Clinton's email practices at the State Department has dogged her for more than a year.
Clinton campaign chairman John Podesta said there was "no evidence of wrongdoing" in the new email review and "no indication this is even about Hillary".
But Mr Comey, who enraged Republicans in the summer when he announced the FBI would not prosecute Mrs Clinton for her handling of official email, in fact said the new trove appeared to be "pertinent" to the Clinton email investigation.
A government official told The Associated Press that the Justice Department had advised the FBI against telling Congress about the new developments in the Clinton investigation because of the potential fallout so close to the election.
Justice officials concluded the letter would be inconsistent with department policy that directs against investigative actions that could be seen as affecting an election or helping a particular candidate.
The email issue threatened to undermine an advantage built by Mrs Clinton over Mr Trump and raised the possibility that the Republican might be able to seize late momentum.
He told a crowd in Golden, Colorado, that the FBI's email review raises "everybody's deepest hope that justice, as last, can be properly delivered".
His crowd cheered Mrs Clinton's email troubles, which Mr Trump has taken to calling the biggest political scandal since Watergate.
The FBI is looking into whether there was classified information on a device belonging to Anthony Weiner, the disgraced ex-congressman who is separated from Clinton aide Huma Abedin.
Mr Comey, in his letter to Congress on Friday, said the FBI had recently come upon new emails while pursuing an unrelated case and was reviewing whether they were classified.