The documents were not among the 30,000 work-related emails turned over to the State Department by her attorneys in December 2014.
The State Department confirmed it has received "tens of thousands" of personal and work-related email materials — including the 14,900 emails found by the FBI — that it will review.
The number of emails provided by the FBI to the State Department for review is much higher than the "several thousand" that FBI Director James Comey said in July were uncovered as part of his agency's investigation.
"We found those additional emails in a variety of ways," Comey explained in July. "Some had been deleted over the years, and we found traces of them on devices that supported or were connected to the private e-mail domain. Others we found by reviewing the archived government e-mail accounts of people who had been government employees at the same time as Secretary Clinton ... Still others we recovered from the laborious review of the millions of email fragments dumped into the slack space of the server decommissioned in 2013."
Meanwhile, the State Department, in response to a Freedom of Information lawsuit, released call logs of top Clinton aide Cheryl Mills, whose name has been attached to efforts to get a Clinton donor placed on a government intelligence advisory board.
One of the callers, Laura Graham, the COO for the Clinton Foundation, called Mills frequently — sometimes several times a day.
“Urgent question as it relates to security and asks to speak with you bf you meet with the PM,” Graham said in a message on Feb. 8, 2012.
Regarding Mills, State Department spokesman Mark Toner said, "Again, we have seen no evidence of any behavior, any relations with the Clinton Foundation that weren't completely aboveboard, and in this case it's likely that what they were dealing with during many of these calls was the immediate aftermath of the Haiti earthquake."
The State Department committed last week to publicly releasing the Clinton emails uncovered by the FBI as part of an existing Freedom of Information Act lawsuit brought by the conservative watchdog group Judicial Watch.
At a status hearing Monday before U.S. District Judge James Boasberg, who is overseeing that case, the State Department presented a schedule for how it will release the emails found by the FBI.
The first group of 14,900 emails was ordered released, and a status hearing on Sept. 23 "will determine the release of the new emails and documents," Boasberg said.
“As we have previously explained, the State Department voluntarily agreed to produce to Judicial Watch any emails sent or received by Secretary Clinton in her official capacity during her tenure as secretary of state which are contained within the material turned over by the FBI and which were not already processed for FOIA by the State Department," Toner said in a statement issued Monday.
"We can confirm that the FBI material includes tens of thousands of nonrecord (meaning personal) and record materials that will have to be carefully appraised at State," it read.
The FBI uncovered the documents as part of its investigation into Clinton's use of private email at the State Department.
"State has not yet had the opportunity to complete a review of the documents to determine whether they are agency records or if they are duplicative of documents State has already produced through the Freedom of Information Act" said Toner, declining further comment.
"We are not sure what additional materials the Justice Department may have located, but if the State Department determines any of them to be work-related, then obviously we support those documents being released publicly as well," said Brian Fallon, the press secretary for the Clinton campaign.
"As we have always said, Hillary Clinton provided the State Department with all the work-related emails she had in her possession in 2014," he said.
At a July news conference announcing the FBI's recommendation that no criminal charges be filed against Clinton, Comey disclosed that investigators found "several thousand work-related emails that were not in the group of 30,000 that were returned by Secretary Clinton to State in 2014."
Three of those several thousand emails were classified at the time they were sent or received, he said.
ABC News' Mike Levine contributed to this story.