The email issue flared up just over a week from election day, threatening Clinton’s lead over Republican Donald Trump.
The FBI said it was looking into whether there was classified information on a device belonging to Anthony Weiner, the disgraced ex-congressman who is separated from long-time Clinton aide Huma Abedin.
Clinton stepped in swiftly, holding a brief news conference in a high school in Des Moines, Iowa.
She challenged FBI director James Comey to release the full details of the new investigation, citing the crucial phase of the White House race.
“We are 11 days out from perhaps the most important national election of our lifetimes. Voting is already under way in our country,” said Clinton.
“So 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.”
Clinton said neither she nor her advisers had been contacted by the FBI about the new inquiry.
The news came as Clinton held the advantage in the presidential race.
Early voting has been under way for weeks and she has a steady lead in preference polls, both nationally and in key battleground states.
The development all but ensures that, even if she wins the White House, the Democrat and several of her closest aides would celebrate a victory under a cloud of investigation.
Trump leapt on the FBI’s disclosure, accusing Clinton of corruption “on a scale we have never seen before”.
“We must not let her take her criminal scheme into the Oval Office,” Trump told a rally in New Hampshire.
Clinton’s campaign was enraged by Comey’s decision to disclose the existence of the fresh investigation in a vaguely worded letter to several congressional leaders.
It was not until hours later that word emerged that the source of the new emails was Weiner, who is under investigation for allegedly sending sexually explicit text messages to a teenage girl.
“It is extraordinary that we would see something like this just 11 days out from a presidential election,” said John Podesta, Clinton’s campaign chairman.
Congressional Republicans have promised years of investigation of Clinton’s private email system.
And that is only one of the email-related controversies she faces in the campaign’s closing days.
The tens of thousands of confidential emails from Clinton campaign insiders that were hacked and then released by WikiLeaks have provided a steady stream of questions about her policy positions and personnel choices, as well as her ties with her husband’s charitable network and post-presidential pursuits.
In his Friday letter to congressional leaders, Comey wrote only that new emails had emerged.
This had prompted the agency to “take appropriate investigative steps” to review the information that may be pertinent to its previously closed investigation.