FBI director James Comey has told Congress the bureau is investigating newly found Hillary Clinton emails.
The latest emails came to light during a separate inquiry into top Clinton aide Huma Abedin’s estranged husband, former congressman Anthony Weiner.
Mr Comey said investigators would determine whether the messages contain classified information.
The FBI has already found the Democratic candidate had sensitive information on a private email server.
The new findings emerged when devices belonging to Ms Abedin and Mr Weiner were seized in an FBI inquiry into whether he sent sexually explicit emails to a 15-year-old girl in North Carolina.
The FBI chief said in his letter to Congress that investigators had discovered the emails “in connection with an unrelated case… that appear to be pertinent to the investigation”.
He said 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”.
In July, Mr Comey said Mrs Clinton’s handling of classified material during her 2009-13 tenure as secretary of state was “extremely careless”, but cleared her of any criminal wrongdoing.
The revelation that she handled sensitive information while breaking federal rules by running her own email server out of her upstate New York home has dogged her campaign since last year.
How big is this?
It could be nothing. It could be everything. And it almost certainly won’t be resolved before Americans head to the polls in just under two weeks.
The letter from Mr Comey to Congress is frustratingly vague. There’s no information, for instance, or how many emails are in question. That will only fuel the rampant speculation already breaking out, with leaks from “government sources” in the coming days sure to fan the flames.
Mrs Clinton’s critics will go on the attack, using the latest news to support their claims that the former secretary of state engaged in malfeasance. Her supporters will spend the next few days in a defensive crouch, trying to assess how bad the damage could be.
What’s certain, however, is that whether this turns out to be a big deal or not, it places the spotlight on all the wrong places for the Clinton campaign. It all but guarantees that even if she wins White House, the early days of her presidency will be dogged by this long-running political imbroglio.
Clinton campaign chairman John Podesta criticised the FBI’s “extraordinary” timing.
He said on Friday: “The (FBI) Director owes it to the American people to immediately provide the full details of what he is now examining.
“We are confident this will not produce any conclusions different from the one the FBI reached in July.”
The revelation 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 Friday afternoon rally in Cedar Rapids, Iowa, she did not respond to the latest developments.
With Clinton team – Our Correspondent’s analysis
What a difference a plane journey makes. When we left Westchester, New York, on Hillary Clinton’s campaign plane, spirits were high amongst her staff.
Her campaign manager, Robby Mook, came to the back of the plane and told reporters that early voting was going in their favour and that Mrs Clinton would even campaign in Republican-leaning Arizona next week.
But Mr Mook added that they weren’t taking anything for granted. He said: “Hillary is superstitious.”
When we landed and wifi internet was restored, the Clinton team first learned the news of a reopened FBI investigation.
She was right to be superstitious.
As we got off the plane, one of her advisers told us: “We are just learning about this at the same time as you are.”
Mrs Clinton stayed on for far longer than usual. Her team were digesting the news and working out how to respond.
When she did walk off the plane, she smiled and ignored shouted questions from the media on the tarmac about the long-running email saga.