Sources have told the Washington Post that the emails were found on a computer belonging to former congressman Anthony Weiner who is under investigation for allegedly exchanging lewd messages with a 15-year-old girl. Weiner is the estranged husband of top Clinton adviser Huma Abedin.
Although Comey, in his letter to Congress, said that whether the emails provide any new information to the Clinton investigation had yet to be determined, Democrats worry the news could sway the election despite no new suggestions of wrongdoing.
Florida is seen as a key battleground state, with an Electoral College victory for Trump seen as being nearly impossible without him winning there.
Trump started the day in Nevada, where he stopped by a non-denominational church in Las Vegas, where he is also held a rally.
Trump quickly seized on Comey's announcement, using it to declare Clinton of being guilty of criminal wrongdoing at campaign events.
Despite the original FBI investigation finding no evidence of criminal wrongdoing by Clinton or her staff and Comey saying the new batch of emails may not be significant, Trump maintained that line of attack today.
"As you've heard it was just announced that the FBI is reopening their investigation into the criminal and illegal conduct of Hillary Clinton," Trump said during his rally in Las Vegas. "Hillary has nobody but herself to blame for her mounting legal problems. Her criminal action was willful, deliberate, intentional and purposeful."
"We never thought we were going to say 'thank you' to Anthony Weiner," Trump said.
THE LATEST
RCP national poll average
Hillary Clinton 47.6 (+4.3)
Donald Trump 43.3
Favourability ratings
Clinton +13.8
Betting odds
Clinton 82
Trump 18
ELECTORAL COLLEGE
270 votes needed to win election
263 safe Clinton (leads by +5)
126 safe Trump (leads by +5)
Battleground states
Leans Clinton
Colorado +4.5, 9 EC
North Carolina +2.9, 15 EC votes
Toss-up Clinton
Nevada +1.7, 6 EC votes
Arizona +0.6, 11 EC votes
Tie
Florida 29 EC votes
Toss-up Trump
Ohio +1, 18 EC votes
Iowa +1.4, 6 EC votes
Leans Trump
Georgia +2.8, 16 EC votes
Texas +4.7, 38 EC votes
Also Trump referred to news that the Justice Department had warned the FBI that Comey's announcement broke protocol, as Justice "trying so hard to protect Hillary Clinton".
Trump is also scheduled to hold campaign events in Colorado and New Mexico.
Until now, the issue of Clinton's use of a private email server while serving as Secretary of State seemed to be receding, and Clinton had sounded increasingly confident as she maintained a lead in most national polls over the past several weeks.
But polls had begun to tighten even before the FBI development. A Washington Post-ABC News tracking poll released today showed Clinton's lead over Trump narrowing to just one percentage point.
The ABC-Post poll also showed that the majority of likely voters remain unmoved by the FBI's announcement, but 34 per cent did say they are less likely to vote for Clinton as a result. Roughly two-thirds of those respondents were Republicans or Republican leaning independent voters.
The Clinton campaign, meanwhile, announced that Tim Kaine would campaign in Arizona later this week, showing that the campaign is still interested in expanding the electoral map even as many in the Democratic Party argue the campaign should retrench and focus on key states.
