The FBI dealt Hillary Clinton's seemingly unstoppable White House campaign a stunning blow Friday by reopening a probe into her use of a private email server while secretary of state.
With just 11 days to go until America votes, Clinton's rival Donald Trump seized triumphantly on the shock decision, declaring her unfit for office in front of a crowd chanting: "Lock her up!"
Clinton was on a flight when the news broke and ignored reporters' shouted questions about the probe as she landed -- smiling and waving -- in Cedar Rapids, Iowa, for campaign events of her own.
Clinton's vice-presidential running mate Tim Kaine, appearing in Tallahassee, Florida, appeared to be caught off guard by the decision, telling reporters: "I've got to read more. I've got to read a little more."
FBI director James Comey dropped the bombshell in a letter to congressional committees investigating allegations that Clinton put US secrets at risk during her time as secretary of state.
Comey had previously told lawmakers the FBI probe into Clinton's unusual decision to use a private server instead of a government email while in office had concluded without finding wrongdoing.
But, writing to "supplement" this assessment, he said that "in connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation."
The FBI will thus take "appropriate investigative steps" to decide whether a new batch of mails contains classified information "as well as to assess their importance to the investigation."
The 70-year-old Trump, trailing in opinion polls both nationally and in the key swing states he must win to secure the White House, seized triumphantly on the news.
"We must not let her take her criminal scheme into the Oval Office," he told cheering crowds at a campaign rally in Manchester, New Hampshire.
"I have 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," he added.
The news took the shine off what should have been a good day for Clinton, with President Barack Obama's Democratic administration announcing stronger than expected economic growth numbers.
While the 69-year-old former first lady holds an overall poll lead and remains on course to become America's first female president, a handful of states -- including Iowa -- could be close-fought.
Both candidates were to be in the battleground of Iowa within hours of each other as they vie to drum up support across the rolling plains of the Midwest.
From there, Clinton was to head to the state capital Des Moines to rally voters.
Trump kicked off the day in New Hampshire, jetting to Maine and was to wrap up with a rally for 5,000 people in an open-air amphitheater in Cedar Rapids.
While Trump draws the biggest crowds, he is failing to pull in donations to match: contributions to his campaign slumped in October.
Both camps filed their campaign accounts Thursday night for the period leading up to October 19. These confirmed the Democrats' overpowering lead in the money game, even though the total sums raised fall short of the records set in 2012.
For the home stretch, Clinton has at her disposal a party war chest almost four times that of her rival: $62 million against $16 million.
When secondary sources of funding are taken into account, the Clinton treasury rises to $172 million against $73 million for the Republican, making it an exceptionally unbalanced race in financial terms, according to Politico.
What does this mean in practice? That American television screens will be awash in Democratic ads in the days leading up to November 8.
The other surprise from the figures reported to the Federal Election Commission: the billionaire Republican donated a measly $31,000 to his own campaign this month -- despite promises to contribute $100 million by Election Day.
The Manhattan real estate mogul -- who claims to be worth $10 billion -- has forked out $56 million of his own money since launching his maverick White House bid last year -- but so far no sign of the other $44 million he pledged to spend.
Trump's White House bid was sent into a tailspin this month by the release of a 2005 video capturing him bragging about the ability to grope women with impunity, followed by a string of accusations of sexual misconduct -- which he denies.
As his campaign faltered, the nominee piled on the defiant rhetoric, claiming the allegations were part of a plot to rig the election, and threatening not to recognize the outcome if he loses.