Pressure on FBI director James Comey has intensified since the latest probe came to light
Donald Trump has praised the FBI for its new investigation into Hillary Clinton's emails, saying "it took guts".
The Democrat was quick to hit back, saying she was confident the bureau would conclude "there is no case here".
Both presidential candidates were speaking at campaign events on Monday, with Mr Trump attending a rally in Michigan while Mrs Clinton addressed crowds in Ohio.
Mr Trump used the opportunity to again seize on the latest revelations, after it was revealed last week that the FBI would examine whether newly discovered emails contained sensitive government information.
He said Mrs Clinton, whose lead in the polls has narrowed this week, had put "the safety of the American people under threat" and was "likely to be under investigation for a very long time".
"Hillary is the one who lied to congress under oath," he said. "Hillary is the one who lied on so many different occasions to the FBI."
"They know Hillary is the one who made 13 phones disappear, some with a hammer.
"Hillary is the one who destroyed 33,000 emails. Hillary is the one who broke the law over and over and over again."
Wow, Twitter, Google and Facebook are burying the FBI criminal investigation of Clinton. Very dishonest media!— Donald J. Trump (@realDonaldTrump) October 30, 2016
Speaking about FBI director James Comey, the billionaire Republican said he wanted to "give credit" to him for opening the fresh investigation.
"I really disagreed with him, I was not his fan," he said, referring to his previous fierce criticism over the decision in July not to recommend prosecution against Mrs Clinton for her private email use.
But Mr Trump said the bureau director had "brought back his reputation".
Pressure on Mr Comey has intensified since the latest probe came to light, with top Democrats demanding the bureau releases "more detailed information".
While not pursuing criminal charges, the FBI concluded in July that Mrs Clinton was "extremely careless" in handling classified information while Secretary of State, with scores of emails on her private internet server containing highly classified information.
Addressing the crowd in Ohio, Mrs Clinton immediately acknowledged the issue, saying she realised people would be wondering what the latest investigation was about.
"And why in the world the FBI would decide to jump into an election with no evidence of any wrongdoing with just days to go", she said.
She said she was not "making excuses" for her use of a personal email address, and that if the FBI wants to investigate emails, "they should look at them."
Mrs Clinton went on to warn voters about the consequences of Mr Trump becoming president, criticising his "casual" attitude to nuclear weapons.