The FBI said it will investigate whether there is classified information in newly discovered emails that appear to be related to its probe of Hillary Clinton's email practices, re-injecting one of the most toxic political issues into the presidential campaign less than two weeks before Election Day.
The new emails came from a sexting investigation of Anthony Weiner, a US official said, speaking only on condition of anonymity because the person was not authorised to comment publicly.
Mr Weiner is the estranged husband of Huma Abedin, a long-time Clinton aide who now is a top official on her campaign team.
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 thinks she is above the law and that she put US security at risk by using her personal email.
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 "willing 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 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.
She made no mention of the FBI development at a rally in Cedar Rapids, Iowa, on Friday, though she warned her supporters "anything can happen" in the campaign's closing days.
She told the crowd that Mr Trump is particularly trying to get women, young people and minorities to stay home - "part of his scorched earth campaign, the last refuge of a bankrupt candidate".
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, was 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 putting an additional 10 million dollars of his personal fortune into his presidential bid, after new federal filings that show Mrs Clinton with an 85 million dollar cash advantage in the final stretch.
The latest fundraising records, which cover the first 19 days of the month, show Mr Trump had given only about 33,000 dollars this month - far less than the two million dollars he typically gives and still 44 million short of the 100 million dollars he has repeatedly promised to contribute over the course of the campaign.
Mrs Clinton's campaign and joint accounts with Democrats had 153 million dollars in the bank as of last week. That is more than double the 68 million dollars the Republican's campaign and partnership committees had on hand.
There are signs that Mrs Clinton's team is at least partly turning to post-election thinking. Vice President Joe Biden has emerged as a surprise name on a list of possible contenders for secretary of state, according to a person familiar with Mrs Clinton's transition planning.
It is unclear how many people are under consideration or whether Mrs Clinton herself has asked for Mr Biden to be vetted for a job she once held. A person familiar with Mr Biden's post-White House plans says the vice president has not been contacted by the Clinton campaign.