Republican presidential candidate Donald Trump has said he believes the FBI may have found the "motherlode" of Hillary Clinton's emails.
At a rally in Grand Rapids, Michigan, he said he hoped the Democratic candidate's 33,000 deleted messages could now be recovered.
The FBI is investigating new emails that may be linked to its probe into Mrs Clinton's private email server.
Mrs Clinton later told a rally she had nothing to hide.
It emerged in March 2015 that Mrs Clinton had been breaking federal rules by operating a private email server while she was secretary of state from 2009-13.
Her lawyers combed through the server and provided the State Department with 30,000 work-related emails.
But her campaign deleted another 33,000 messages, saying they were personal in nature.
FBI Director James Comey concluded in July that Mrs Clinton had been "extremely careless" in handling classified information, but there were no grounds for any charges.
But on Friday, he told Congress he had learned of fresh emails which might be "pertinent" to its previous inquiry and renewed the FBI probe.
The latest emails were found in a separate investigation into allegations that former congressman Anthony Weiner sent illicit text messages to a 15-year-old girl in North Carolina. Mr Weiner is married to top Clinton aide, Huma Abedin.
The FBI has reportedly obtained a warrant to search the cache of emails belonging to Ms Abedin, which are believed to have been found on her estranged husband's laptop.
There are reportedly 650,000 emails to search through on the laptop, but it is unclear who sent or received the emails or what they were about.
"That's the motherlode," said Mr Trump. "I think you're going to find the 33,000 (emails) that are missing."
He added: "Thank you, Huma. Thank you, Huma. Good job, Huma. Thank you, Anthony Weiner."
But Mrs Clinton told a rally in Kent, Ohio, that the FBI was welcome to check her aide's emails.
"They should look at them and I'm sure they'll reach the same conclusion they did when they looked at my emails for the past year: there is no case here."
Democrats have angrily demanded that the embattled Mr Comey rapidly make public what the agency knows about the new email trove.
The White House said on Monday it would neither defend nor criticise the FBI decision.
Obama spokesman Josh Earnest said the president did not suspect Mr Comey of trying to secretly influence the election through his announcement on Friday of the inquiry.
Mr Earnest said Mr Comey is "a man of integrity, a man of character, a man of principle and he has a very difficult job".
On Sunday, Democratic leader in the US Senate Harry Reid accused Mr Comey of violating an act which bars officials from influencing an election by revealing the bureau was investigating emails possibly linked to Mrs Clinton.