The FBI has interviewed top Hillary Clinton aide Huma Abedin as part of its investigation into the former secretary of state’s use of a private server, a source close to the investigation told Fox News.

The source did not say when the interview took place or when the FBI’s investigation might be completed. 

The FBI and Justice Department have been investigating whether sensitive information that flowed through Clinton's email was mishandled.

Abedin was one of Clinton's closest aides during her tenure as the nation's top diplomat, serving as deputy chief of staff. The interview of Abedin, part of Clinton's inner circle, is a possible indication that the investigation might be in its final stages. It was not immediately clear whether other aides have been interviewed, or when or if Clinton herself might be questioned. 

According to a Los Angeles Times report, the Abedin interview was held April 5 in Washington.

"From the start, Hillary Clinton has offered to answer any questions that would help the Justice Department complete its review, and we hope and expect that anyone else who is asked would do the same," Clinton campaign spokesman Brian Fallon said in a statement. "We are confident the review will conclude that nothing inappropriate took place."

On Wednesday, a federal judge in Washington also said he may order Clinton, the Democratic presidential front-runner, to testify under oath about whether she used a private email server to evade public records disclosures.

The order from U.S. District Judge Emmet Sullivan granted a request from the conservative legal advocacy group Judicial Watch to question six current and former State Department staffers, including Abedin, about the creation and purpose of the private email system.

Clinton has said the use of a private email server was a mistake, but said what she did was allowed by the State Department. She has dismissed the idea that she could face legal trouble over the use of the server, saying in response to a debate question in March about the prospect of a federal indictment, "It's not going to happen."

The Associated Press contributed to this report.