The FBI has been under pressure to provide details of a renewed investigation into Hillary Clinton’s use of private e-mail while she was secretary of state.
FBI Director James Comey notified Congress on October 28 -- 11 days before the presidential election -- that the agency is looking into a batch of new e-mails.
The Democratic candidate called the move "unprecedented" and "deeply troubling," while her Republican opponent, Donald Trump, praised the FBI's decision.
In his letter to Congressmen, Comey said the FBI learned of fresh e-mails which appeared to be "pertinent" to its previous inquiry into Clinton's use of a private server when she was the top U.S. diplomat in the Obama administration.
He did not explain how.
In a memo sent later to FBI staff, Comey acknowledged "we do not ordinarily tell Congress about ongoing investigations," adding that he felt an "obligation" to do so given that he had previously testified that the FBI investigation was complete.
The FBI chief has been heavily criticized by Clinton and her campaign team for his decision to make the information public so close to polling day.
"It's not just strange, it's unprecedented,” Clinton told supporters in Florida on October 29. “And it is deeply troubling because voters deserve to get full and complete facts.”
"So we've called on Director Comey to explain everything right away, put it all out on the table," Clinton also said.
John Podesta, who heads her presidential campaign, said the information provided by Comey was "long on innuendo" and "short on facts."
Podesta urged Comey to "come forward and give those answers to the American public" about the exact nature of the FBI's latest review of e-mails.
Campaigning in Colorado, Trump said the issue was the “biggest political scandal” in the United States since Watergate, which brought down President Richard Nixon.
Clinton's "criminal action was willful, deliberate, intentional, and purposeful," said the real estate tycoon, who is himself dogged by scandal over alleged sexual misconduct. "Hillary set up an illegal server for the obvious purpose of shielding her criminal conduct from public disclosure and exposure."
In recent weeks, Trump has been running behind Clinton in most public opinion polls.
Meanwhile, four senior Democratic senators called on the FBI and the Justice Department to provide more information by October 31 about investigative steps underway.
Senators Benjamin Cardin of Maryland, Thomas Carper of Delaware, Patrick Leahy of Vermont, and Dianne Feinstein of California made the call in a letter sent on October 29 to Comey and Attorney General Loretta Lynch.
The FBI has already established that Clinton, who served as secretary of state from 2009 to 2013, had held classified information on a private e-mail server.
In July, Comey said Clinton had been "extremely careless" in her handling of sensitive material during her tenure as Washington’s top diplomat, but cleared her of any criminal wrongdoing.
The latest e-mails were found as part of an unrelated investigation into Anthony Weiner, the estranged husband of one of Clinton’s top aides, Huma Abedin.
Devices belonging to them were seized in an FBI probe into illicit messages Weiner, a former congressman, is alleged to have sent to a 15-year-old girl.