Why now, ask stunned political observers - and why the FBI?
Hillary Clinton has led a furious Democratic party backlash against the FBI's decision to investigate a new batch of her staff's emails just 10 days before the presidential election - a move that has breathed new life into the faltering campaign of her rival, Donald Trump.
James Comey, director of the FBI, yesterday faced a torrent of criticism for his dramatic late intervention, which broke the FBI's usual protocol of keeping its ongoing inquiries quiet at a time when they could change the trajectory of a presidential race. He stood accused of betraying the bureau's political neutrality, and was last night under growing pressure to make public all he knows about the emails.
The Clinton campaign was said to be livid with the director - while a jubilant Trump seized on a potential lifeline, describing the Democratic candidate's handling of classified information as a scandal "bigger than Watergate".
In a bombshell announcement on Friday, Comey said the FBI was looking into whether there was classified information on a device belonging to Anthony Weiner, the estranged husband of Huma Abedin, one of Clinton's closest aides. The bureau uncovered the emails during an investigation of Weiner, a disgraced ex-congressman accused of sending sexually explicit text messages to a teenage girl. In July, the FBI closed an investigation into Clinton's use of a private email server when secretary of state. Comey said Clinton and her aides had been "extremely careless" but not criminal in using the system for communications about government business.
An angry Clinton told a press conference in Des Moines, Iowa, on Friday night: "The American people deserve to get the full and complete facts immediately. The director himself has said he doesn't know whether the emails referenced in his letter are significant or not."
The content of the messages is unknown - and may well not be before election day.
"Right now, your guess is as good as mine, and I don't think that's good enough," Clinton said.
The decision by Comey, a Republican appointed by Barack Obama three years ago, appears to contradict department of justice guidelines that discourage any actions close to an election with potential to influence the outcome. The New Yorker magazine's website reported that Comey went against the advice of Attorney General Loretta Lynch.
Matthew Miller, who served at the department of justice under Attorney General Eric Holder, told the Observer: "It was an unacceptable breach of years of department of justice practice and precedent. The department goes out of its way not to take any action close to an election that could influence the outcome. The FBI's reputation for independence and integrity is really at the core of their ability to do their job."
Comey's decision to make an unprecedented televised statement at the end of the Clinton investigation was "the original sin here", he said, but this was "by far the most serious breach of all".
Washington's political establishment was said to be stunned at the timing of Comey's letter informing Congress of additional "investigative steps".
Howard Dean, former Democratic national committee chair tweeted: "Comey tried to do what he thought was right. He botched the presentation and may have destroyed the credibility of the FBI forever."
In a reference to the alleged role of Russia's President Putin in hacking Clinton campaign emails, Dean added: "Ironically, Comey put himself on the same side as Putin."
Dianne Feinstein, the Democratic senator for California, said: "The FBI has a history of extreme caution near election day. Today's break from that is appalling."
Some commentators speculated that Comey felt caught in a bind: if he waited until after the election, or if it leaked through back-channels, he would have been accused of a cover-up. In an internal email to FBI staff, he said he was trying to strike a balance between keeping Congress and the people informed and not creating a misleading impression about the emails, whose significance is yet unknown.
"In trying to strike that balance, in a brief letter and in the middle of an election season, there is significant risk of being misunderstood, but I wanted you to hear directly from me about it," he wrote.
It was unclear if his announcement will affect the election outcome. Early voting is under way in 37 states, and nearly 17m votes have already been cast. Clinton has a healthy lead in most opinion polls.
"I think people a long time ago made up their minds about the emails," she said at her press conference. "And now they are choosing a president."
But Carl Bernstein, who with Washington Post colleague Bob Woodward broke the Watergate story that brought down Richard Nixon, told CNN: "We don't know what this means yet - except that it's a real bombshell.
"And it is unthinkable that the director of the FBI would take this action lightly, that he would put this letter forth to the Congress of the United States, saying there is more information out there about classified emails, unless it was something requiring serious investigation."
Trump, whose own campaign has been plagued by one scandal after another, pounced on the FBI's disclosure, accusing Clinton of corruption "on a scale we have never seen before".
He said: "This is bigger than Watergate... we must not let her take her criminal scheme into the Oval Office... Perhaps, finally, justice will be done."
He added: "The FBI would never have reopened this case unless it was a most egregious offence."