Meanwhile, a newly emboldened Donald Trump is seizing on the discovery of a trove of new emails that may be pertinent to the FBI’s investigation and trying to open new paths to victory by campaigning in traditionally Democratic states.
Mrs Clinton, who was set to campaign across Ohio on Monday, vowed over the weekend that she would not be “knocked off course” in the election’s final days by the discovery of new emails in an unrelated sexting investigation. It is unclear what is contained in the emails or if any of them was sent or received by Mrs Clinton herself.
“I’m not stopping now, we’re just getting warmed up,” shed declared during a packed rally with gay and lesbian supporters in battleground Florida on Sunday. “We’re not going to be distracted, no matter what our opponents throw at us.”
Mr Trump, who had been trailing Mrs Clinton nationally and across key battleground states, campaigned with new vigour over the weekend as he seized on the news in an effort to boost his struggling candidacy. On Monday he was heading for a pair of rallies in MIchigan - a state that last voted for the Republican nominee for president in 1988.
“The polls have come out and they have been amazing, even before the big blow-up on Friday,” Mr Trump told a crowd of thousands packed into an airport in Albuquerque, New Mexico - another traditionally Democratic state that Mr Trump said on Sunday night he believes he can win.
“Traditionally, you understand, Republicans aren’t quite there, right?” he told the crowd. “But this is a Republican who is there and we’re going to win this thing.”
Mrs Clinton’s advisers and fellow Democrats, furious over the vague letter sent by FBI director James Comey to Congress on Friday, have been pressuring him to release more details about the emails, including whether Mr Comey had even reviewed them himself.
The emails were found on a computer that appears to belong to disgraced former New York Representative Anthony Weiner, the estranged husband of Huma Abedin, one of Mrs Clinton’s closest advisers.
A law enforcement official confirmed late on Sunday that investigators had obtained a search warrant to begin the review of Ms Abedin’s emails on Mr Weiner’s computer. The official said investigators would move expeditiously, but would not say when the review might be complete.
Tim Kaine, Mrs Clinton’s running mate, said Mr Comey owed it to the public to be more forthcoming about the emails under review by the FBI with the days ticking down to the November 8 election.
Calling Mr Comey’s announcement “extremely puzzling”, Mr Kaine said that if Mr Comey “hasn’t seen the emails, I mean they need to make that completely plain”.
Mr Comey’s actions on Friday have stirred up the White House race, energising Mr Trump as polls had shown him sliding and unnerved Democrats already worried about the presidency and down-ballot congressional races.
In a letter to Congress on Friday, Mr Comey said the FBI had recently come across new emails while pursuing an unrelated case and was reviewing whether they were classified.
Federal authorities in New York and North Carolina are investigating online communications between Mr Weiner and a 15-year-old girl.
A law enforcement official said on Sunday that FBI investigators in the Weiner sexting probe knew for weeks about the existence of the emails that might be relevant to the Clinton email investigation.
Mr Comey said he was briefed on Thursday about that development and told Congress on Friday that investigators had found the emails. A second law enforcement official also said the FBI was aware for a period of time about the emails before Mr Comey was briefed, but the second official was not more specific.
Mr Trump has praised Mr Comey for his decision, declaring that he believes justice will finally be served.
Trump campaign manager Kellyanne Conway said Mr Comey was in “an impossible spot” when he acknowledged the FBI was looking into the messages. “Had he sat on the information, one can argue that he also would be interfering in the election,” by failing to disclose the review, she said.
The controversy over Mrs Clinton’s email practices while she served as secretary of state has dogged her for more than a year.