Nick Merrill, a Clinton press secretary, said they are working with the FBI to quickly resolve the issue.
"She did not send nor receive any emails that were marked classified at the time," Merrill told CNN. . "We want to ensure that appropriate procedures are followed as these emails are reviewed while not unduly delaying the release of her emails. We want that to happen as quickly and as transparently as possible."
The Washington Post reported that the bureau has contacted Platte River Networks, a Denver-based technology company that managed the system. Clinton's setup included several private email addresses and a private server located at her Chappaqua, N.Y. home.
According to the Post, the bureau is not investigating Clinton herself, but rather the security behind the unusual email setup.
Clinton's lawyer David Ken¬dall is also the subject of an FBI investigation regarding the security of a thumb drive he had that contained Clinton's emails.
"The government is seeking assurance about the storage of those materials. We are actively cooperating," Ken¬dall told the Post.
Platte River Networks reportedly took over management of Clinton's email system in 2013 after years of having low-level Clinton staffers and an IT specialist with the State Department run it.
That specialist, Bryan Pagliano, served as Clinton's 2008 presidential campaign IT director before the State Department hired him in 2009. He managed Clinton's email setup, reports the Post, and had to make several trips to Chappaqua to repair the system.
One such repair was needed when the server went down after Hurricane Sandy hit the New York area in 2012. At that point, reports the Post, Clinton and her team decided they needed to seek an outside company to keep the system secure and working as designed.
Further, the Post reports that Clinton's email server was first installed during her 2008 presidential campaign. It was brought in to replace one used by former President Bill Clinton because that one was too small to handle Clinton's email thanks to her position in the cabinet.
Clinton's email controversy began last year, when investigators looking into what happened during the 2012 Benghazi attacks noticed they were missing correspondence from Clinton. It spiraled out of control from there, and the scandal has dogged Clinton for months.
Clinton, who announced her candidacy for president in April, claimed she used the email setup so she could get away with using one phone instead of a personal one and one provided by the State Department.
Critics have said the system was vulnerable to hackers because it was outside of the government's secure servers.
The results of a poll released Thursday, meanwhile, show Clinton's favorability rating has been sliding since 2012 and is now at 44 percent.