Donald Trump has accused the FBI of impropriety for declaring that his rival Hillary Clinton should not face criminal charges, as Prime Minister Theresa May called for a "calm and measured" presidential vote.
The candidates are entering their final full day of campaigning after FBI director James Comey announced, just two days before Tuesday's election, that the Democratic hopeful was in the clear over her handling of classified information while US secretary of state.
Mr Comey's announcement on the newly-discovered tranche of emails caused relief to spread through the Clinton campaign after the investigation threatened to derail her lead over Republican rival Donald Trump.
But Mr Trump said: "Hillary Clinton is guilty. She knows it. The FBI knows it. The people know it."
The unprecedented chaos came after Mrs May said she would prefer the US election to be held in a "calm and measured" fashion.
She was careful to avoid giving any indication of her preferred winner in the Trump-Clinton showdown.
As home secretary, Mrs May told a Commons committee last year that Mr Trump's comments about barring Muslims from the US were "divisive, unhelpful and wrong".
But she did not respond when asked, during a visit to India, whether she stood by her remark.
The PM insisted that the choice of a US president was a matter for the American people.
But asked about the feverish atmosphere amid which the contest has played out, she said: "I take a simple view about the way in which I like to see campaigns conducted. I like to see them conducted in a calm and measured way with proper consideration of the issues.
"As to the result of the election, it's up to the American people and we'll know that very soon."
In his letter to congressional politicians on Sunday, Mr Comey said the FBI "worked around the clock" on the latest emails.
But investigators found no reason to change its earlier assessment that Mrs Clinton should not be charged for her handing of classified information.
Jennifer Palmieri, Mrs Clinton's communications director, said: "We're glad this matter is resolved."
But Mr Trump declared that his opponent is "protected by a rigged system" after landing in Minnesota moments after the announcement.
Then, during a rally in the Detroit suburbs, he insisted it would have been impossible for the FBI to examine the emails in such a brief time.
He added that Mrs Clinton is "guilty" and the "FBI knows it".