Federal Bureau of Investigation Director James Comey on Friday said that the agency would investigate additional emails that have surfaced related to Hillary Clinton's use of a personal email server to determine whether they contain classified information, adding that it is unclear how significant the new materials may be.
The new probe, which comes barely 11 days before the United States votes for its new president, is a blow for Clinton who is the Democratic Party's candidate and the front-runner in opinion polls.
In a letter to several US congressional committee chairmen, the FBI Director said that he "cannot predict how long it will take us to complete this additional work."
"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Comey wrote. "I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information as well as to assess their importance to our investigation."
The letter from Comey comes nearly four months after the FBI director announced the Bureau would not recommend criminal charges against Clinton for what it dubbed "extreme careless" use of a private email server while secretary of state.
The FBI director said then: "Although there is evidence of potential violations of the statutes... our judgment is that no reasonable prosecutor would bring such a case."
The Clinton campaign did not immediately respond to a request for comment.
The FBI's decision to reopen the investigation was welcomed by Republican presidential nominee Donald Trump as a "big day" soon after the announcement.
The controversial billionaire from New York departed from his prepared speech as he landed for his rally in Manchester in New Hampshire.
"I have a breaking news for you," he told his cheering supporters, who waited for him for hours.
"FBI has just sent a letter to the Congress that they have found new emails pertaining to the former Secretary of State Hillary Clinton's email investigation. And they are reopening the case into her illegal and criminal conduct that threatened the national security of the United States," he said.
"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," Trump said and hoped that "perhaps finally justice would be done."
"This is a big day," he said alleging that the email scandal is bigger than Watergate. " Hillary Clinton's corruption is on a scale that we have never seen before."
Other Republican leaders also welcomed the decision.
House Judiciary Committee Chairman Bob Goodlatte said the move shows Clinton and her team "committed wrongdoing and jeopardised national security."
House of Representative Speaker Paul Ryan said Clinton has nobody but herself to blame. "She was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information," he said.
"This decision, long overdue, is the result of her reckless use of a private email server, and her refusal to be forthcoming with federal investigators. I renew my call for the Director of National Intelligence to suspend all classified briefings for Secretary Clinton until this matter is fully resolved," Ryan said.
Trump's campaign manager Kellyanne Conway called this a great news. "A great day in our campaign just got even better. FBI reviewing new emails in Clinton probe," she tweeted.
Photograph: Reuters