Clinton, who is likely to win the race to become the Democratic Party's candidate in November's presidential election, has said she was willing to be interviewed in connection with the FBI's investigation into whether any laws were broken as a result of the email arrangement.
More than 2,000 emails sent and received by Clinton while working as President Barack Obama's top diplomat include classified information, which the government bans from being handled outside secure, government-controlled channels.
Clinton has said she did not send or receive any information that was marked as classified and that she expects to be exonerated, a point her spokesman echoed on Thursday.
"From the start, Hillary Clinton has offered to answer any questions that would help the Justice Department complete its review," the Clinton campaign said in a statement to CNN. "We are confident the review will conclude that nothing inappropriate took place."
David Kendall, Clinton's lawyer, declined to comment on the CNN report, as did Melanie Newman, a spokesperson for the U.S. Justice Department. The FBI had no immediate comment.
CNN said those interviewed included Huma Abedin, a senior aide to Clinton and the vice-chairwoman of Clinton's presidential campaign.
Lawyers for Abedin and other Clinton aides did not immediately respond to a request for comment.