The FBI is likely to interview Hillary Clinton in the next few weeks about her use of a private email server while she was U.S. secretary of state and have already interviewed some of her aides, CNN reported on Thursday.

The nine-month investigation into whether laws were broken as a result of the server kept in her New York home has overshadowed Clinton's campaign to become the Democratic Party's candidate in November's presidential election.

With only a few states left to vote in primary elections, she retains a commanding lead over her rival, U.S. Sen. Bernie Sanders of Vermont.

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 has accused the State Department and other government agencies of "over-classifying" her emails after a judge ordered them released to the public. She has said she expects to be exonerated by the FBI, a point her campaign staff echoed on Thursday.

"We are confident the review will conclude that nothing inappropriate took place," the campaign told CNN.

David Kendall, Clinton's lawyer, and Melanie Newman, a spokesperson for the U.S. Justice Department, which oversees the FBI, both declined to 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 respond to questions.

The timeline in the CNN report appeared to contradict answers Clinton gave in an interview on Tuesday, in which she told MSNBC's Andrea Mitchell that neither she nor her representatives had yet been contacted by the FBI.

Asked about this, Clinton spokesman Brian Fallon suggested the definition of words Mitchell used in her question were open to interpretation.

"What does representatives mean to you, sir?" he said in an email, but declined to elaborate.

Donald Trump, the Republican Party's presumptive presidential nominee, and other Republicans have used the FBI inquiry to attack Clinton's integrity.

"These FBI interviews are another reminder of the gross negligence Hillary Clinton displayed as Secretary of State," Reince Priebus, the Republican National Committee chairman, said in a statement.