FBI director James Comey has told Congress the bureau is investigating newly found Hillary Clinton emails.
Investigators discovered the 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 sensitive information on a private email server.
Comey had previously called Mrs Clinton’s handling of classified material during her 2009-13 tenure 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 be 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.
Clinton is five points ahead of her Republican rival Donald Trump, according to a Real Clear Politics average.
She has yet to respond to the latest developments, which occurred during her flight to Cedar Rapids, Iowa, where she is holding a rally.
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 threatened 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.
“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 declared.
