Donald Trump. AP Donald Trump on Sunday said Hillary Clinton was "being protected by a rigged system," hours after the FBI once again cleared her in its investigation into her use of a private email server.

"Right now she is being protected by a rigged system. It's a totally rigged system," the Republican presidential nominee said at a rally in Minnesota. "I've been saying it for a long time."

The FBI director, James Comey, said on Sunday that his team had been "working around the clock" to process the newly discovered emails since he announced a little over a week ago that the investigation had been renewed.

"We reviewed all communications that were to or from Hillary Clinton while she served as secretary of state," Comey wrote in his letter to Congress. "Based on our review, we have not changed our conclusions that we expressed in July with respect to Secretary Clinton."

Trump, however, cast doubt on the FBI director's methods.

"You can't review 650,000 new emails in eight days," Trump said. "You can't do it, folks. Hillary Clinton is guilty. She knows it, the FBI knows it, the people know it."

The New York businessman said it was now up to voters to hold Clinton accountable.

"Now it is up to the American people to deliver justice at the ballot box on November 8," he said. "It's unbelievable. It's unbelievable what she gets away with."

Trump had been using the latest FBI investigation to attack Clinton. In the past week, he and his surrogates used every opportunity they got to suggest the investigation would ultimately lead to an indictment that could thrust the country into an unprecedented constitutional crisis should she be elected.

With just two days to go until Election Day, both Clinton and Trump were furiously crisscrossing the US in a frantic last-minute push to turn out the vote in key battleground states.

Polls in recent days have increasingly tightened, though analysts still favor Clinton to emerge as the victor Tuesday night.