FBI director James Comey owes it to the American people to immediately provide the full details of what his agents are examining related to the Democratic nominee Hillary Clinton, her campaign chairman said.
John Podesta was responding on Friday to a letter from Mr Comey informing Congress that the FBI is investigating whether there is classified information in new emails that have emerged in its probe of Mrs Clinton's private server.
Mr Podesta said Mr Comey's letter was extraordinary, coming just 11 days away from the presidential election.
Mr Podesta said he is confident the new information will produce no conclusions different from the one the FBI reached in July, when the bureau closed its investigation without finding evidence to support criminal charges.
Donald Trump immediately pounced on the turn of events, seeing an opportunity to press the argument he has long tried to make against Mrs Clinton: that she uses her public positions for personal gain.
The Republican nominee told cheering supporters at his first campaign rally of the day that he has "great respect" for the fact that the FBI and the Department of Justice are now "wiling to have the courage to right the horrible mistake that they made" in concluding the investigation earlier.
Mr Trump said of Mrs Clinton: "We must not let her take her criminal scheme into the Oval Office. ... This is bigger than Watergate."
The White House said it had no advance warning about Mr Comey's decision to investigate new emails.
Spokesman Eric Schultz said the White House learned of Mr Comey's decision from news reports.
Mr Schultz said President Barack Obama expects the FBI will follow the facts wherever they lead.
The Friday afternoon disclosure raises the possibility of the FBI reopening the criminal investigation involving the Democratic presidential nominee, which the agency said was complete in July.
In a letter sent on Friday to congressional leaders, FBI Director James Comey said that new emails have emerged, prompting the agency to "take appropriate investigative steps" to review information that flowed through the private email server Mrs Clinton used while serving as secretary of state.
Mrs Clinton's email use has been one of the biggest vulnerabilities in her campaign for the White House. Even if she wins, Republicans have vowed the issue will follow her, promising continuing investigations.
Mrs Clinton's campaign did not immediately respond to the news. She ignored shouted questions from reporters as she walked off her plane in Cedar Rapids, Iowa.
But her campaign was fending off other political problems as well, still trying dismiss the revelations in thousands of messages stolen from the private account of a top Clinton aide, part of a hack the Democratic campaign has blamed on the Russians.
Correspondence made public on Wednesday showed long-time Bill Clinton aide Doug Band describing overlapping relationships of the Clintons' global philanthropy and the family's private enrichment.
"These are illegally stolen documents," Clinton campaign manager Robby Mook told reporters on her campaign plane.
"We're not going to spend our campaign fighting back what the Russians want this to be about."
So far, the email-related controversies have not seemed to hurt her campaign in the final weeks.
Recent surveys show Mrs Clinton retaining her lead in national polls and making gains in some swing states.
Her campaign announced plans to hold a rally in Arizona next Wednesday, a traditionally red state put in play by Mr Trump's deep unpopularity among minority voters, Mormons and business leaders.
Feeling confident, she has begun focusing on helping Democrats win control of the Senate, expand their margin in the House and lay the groundwork for future victories in demographically-shifting states like Arizona.
Her campaign will also get a boost from President Barack Obama, who will hold an evening rally in Orlando, a key battleground area of the crucial swing state of Florida.
White House officials said Mr Obama will be travelling to boost Mrs Clinton nearly every day until Election Day, reflecting his rising popularity ratings.
Mr Trump, meanwhile, is holding Friday events in New Hampshire, Iowa and Maine, one of two states that split their electoral voters by congressional district.
Facing an increasingly narrow path to the 270 electoral votes needed to win the White House, his campaign is shooting for one of the traditionally Democratic state's four electoral votes in the more rural, conservative second district.
His campaign said the billionaire businessman is plunging an additional $10 million of his personal fortune into his presidential bid, after new federal filings that show Mrs Clinton with an $85 million cash advantage in the final stretch.
