Donald Trump cries foul as FBI clears Hillary Clinton in email probe

Republican US presidential nominee Donald Trump has accused the FBI of impropriety after it once again exonerated his rival Hillary Clinton of criminal conduct on her emails.

With just one day until the election, Democrat Hillary Clinton heads into the race with new momentum after the FBI's announcement that no criminal charges were forthcoming in the probe of her email practices.

FBI Director James Comey sent shockwaves through the race by telling Congress on Sunday that investigators had worked "around the clock" to complete a review of newly discovered emails and found no reason to change their July finding that Clinton was not guilty of criminal wrongdoing in her use of a private email server while secretary of state.

However, Reuters news agency  claims that it remains uncertain whether the announcement came in time to change minds or undo any damage from days of Republican attacks on Clinton as corrupt. Tens of millions of Americans had cast early votes in the 10 days since Comey first told Congress of the newly discovered emails.

At a rally in the Detroit suburbs, Trump insisted it would have been impossible for the FBI to review what has been reported to be as many as 650,000 emails in such a short time.

"Right now she's being protected by a rigged system. It's a totally rigged system. I've been saying it for a long time," he told supporters in Sterling Heights, Michigan.

Both Clinton and Trump will spend the day racing across a handful of key battleground states that could swing Tuesday's election, which polls show is extremely close but tilting toward Clinton.

On Monday, Trump will hit five battleground states - Florida, North Carolina, Pennsylvania, New Hampshire and Michigan - and closes with a late-night rally in Grand Rapids, Michigan.

Clinton will make two stops in Pennsylvania and visit Michigan before wrapping up with a midnight rally in Raleigh, North Carolina. Earlier, she will appear at an evening rally at Philadelphia's Independence Hall with President Barack Obama and first lady Michelle Obama, as well as rock star Bruce Springsteen.