Hillary Clinton has criticised FBI Director James Comey's eleventh-hour announcement of the reopening of an investigation into her use of a private email server as "unprecedented and deeply troubling".
The Democratic presidential candidate on Saturday and called on Comey to provide the public with more information without delay.
Speaking to supporters in Daytona, Florida, she said "if you're like me, you probably have a few questions" about Comey's letter informing Congress the FBI was reviewing more emails in the case.
"Voters deserve to get the full and complete facts," she said, calling on Comey to "to explain everything right away, put it all out on the table".
The FBI has not said what the emails contain or even whether they were sent by or to Clinton. Comey told Congress it was not yet clear whether the material was significant, and that he did not know how long a full review would take.
The emails appear to be linked to investigations of disgraced former New York congressman Anthony Weiner, also a Democrat and the husband of Hillary Clinton's closest aide, Huma Abedin. Weiner is implicated in a sex scandal, and in the meantime he and Abedin have separated.
US media outlets reported on Saturday that Comey defied the wishes of his boss, Attorney General Loretta Lynch, when he made public the agency's review of the new emails.
Lynch thought it inappropriate to make public ongoing procedures with possible implications for a presidential candidate less than two weeks before the election, according to the reports by CNN and the New Yorker, which cited government officials.
Lynch told Comey to follow the long-standing practice of avoiding actions that could influence the outcome of the election.
Clinton called Comey's decision to defy Justice Department practice "strange," "unprecedented and deeply troubling".
On Friday, Comey told Congress in a letter that the FBI was reviewing new emails that could be relevant to its investigation into the Democratic presidential candidate's handling of classified information during her tenure as secretary of state.
The federal law enforcement chief said he felt obligated to make the information public.
Comey is a Republican, but up to now he has been seen as non-partisan in his office.
Following a nearly year-long investigation, Comey found in July that Clinton had been "extremely careless" in her handling of classified information - but had not broken the law.
The revelation comes less than two weeks before the US presidential election pitting Clinton against Republican Donald Trump, who has made Clinton's trustworthiness related to the email scandal a key part of his argument against her.