(BBC) FBI director James Comey has said the bureau is reopening its investigation into Hillary Clinton’s emails.
Investigators have discovered new emails “in connection with an unrelated case… that appear to be pertinent to the investigation”, he said.
Mr Comey said investigators would determine whether the emails contain classified information.
The FBI has already found the Democratic candidate had classified information on a private email server.
Mr Comey had previously called Mrs Clinton’s handling of classified information during her time as secretary of state “extremely careless”, but cleared her of any criminal wrongdoing.
The FBI chief said in his letter to Congress he “cannot yet assess whether or not this material may significant, and I cannot predict how long it will take us to complete this additional work”.
The potential bombshell comes just 11 days before Americans go to the polls in the presidential election.
Mrs Clinton is five points ahead of her Republican rival Donald Trump, according to a Real Clear Politics average.
At a rally in Manchester, New Hampshire, Mr Trump told a cheering crowd: “They’ve reopened the case into her criminal and illegal conduct that threatens the security of the United States of America.
“Hillary Clinton’s corruption is on a scale we’ve never seen before. We must not let her take her criminal scheme into the Oval Office.”
The revelation that she broke federal rules by running her own email server out of her home in upstate New York and handled sensitive information has damaged her trustworthiness ratings with voters.
The House Judiciary Committee Chairman Bob Goodlatte said: “The more we learn about Secretary Clinton’s use of a private email server, the clearer it becomes that she and her associates committed wrongdoing and jeopardised national security.
“Now that the FBI has reopened the matter, it must conduct the investigation with impartiality and thoroughness. The American people deserve no less and no one should be above the law.”
Kellyanne Conway, a spokesman for Mr Trump, said on Twitter, that “a great day in our campaign just got even better”.
Paul Ryan, the highest-ranked elected Republican, called the decision “long overdue”.
“Yet again, Hillary Clinton has nobody but herself to blame,” the House of Representatives Speaker and Wisconsin congressman said.
“She was entrusted with some of our nation’s most important secrets, and she betrayed that trust by carelessly mishandling highly classified information.”