On October 28 - less than two weeks before Election Day - Comey dropped a bombshell by informing Congress the FBI was looking once more into Clinton's controversial email practices as secretary of state after new messages were uncovered.
His second letter, on November 6, just three days before the election, stated that the FBI's review of the new emails had uncovered no wrongdoing, and that the bureau had not changed its July recommendation not to charge Clinton.
While the first letter reopened old wounds and reminded voters of the controversy, the second letter, according to Clinton, had the perverse effect of energising rival Donald Trump's base, Quartz reported.
The letters came just as Clinton was building momentum following strong performances at all three presidential debates, and after the release of a damaging recording of Trump boasting about groping women.
"After the third debate we felt so good about where we were," Clinton said, according to Quartz's source, who chose to remain anonymous.
The campaign analysis "showed that we were up in all but two of the battleground states, where we were tied or one point behind. In Arizona we were even. We felt real wind at our back." But she said the FBI's last-minute role was too much to "overcome," a donor on the call told CNN.
In July Comey said that the FBI would not recommend criminal charges against Clinton for handling classified documents and sensitive information on a private email account.
Comey however slammed the practice as "extremely careless." The FBI chief came under fierce criticism for effectively reopening the Clinton probe so close to the November 8 vote, without knowing whether the new emails were relevant.
