Federal investigators have interviewed top aides to Hillary Clinton about her use of a private email server, the latest advance in an ongoing investigation into whether her email practices as secretary of state may have compromised classified information, according to two people familiar with the matter.

The interviews, of close aides including Huma Abedin, have been conducted by FBI agents, lawyers from the Justice Department's National Security division, and prosecutors from the U.S. Attorney's Office in Alexandria, Va.

There's no sign yet that a federal grand jury has been convened in the case, and Clinton herself has not yet been interviewed. Clinton, who's running for the Democratic presidential nomination, says she is cooperating with the probe.

"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's press secretary Brian Fallon said. "We are confident the review will conclude that nothing inappropriate took place."

The attorney general and the FBI director have offered no deadline for their review.

The interviews with Clinton aides were first reported by CNN.

In an interview with NPR last week, Attorney General Loretta Lynch said, "our goal is to be thorough because we want to make sure that in fact we have looked at everything we need to look at before we come to any final conclusions, whichever way."

The federal investigation continues, even as dozens of civil lawsuits involving open records laws move their own way through the courts.

Earlier this week, U.S. District Judge Emmet G. Sullivan paved the way for the conservative watchdog group Judicial Watch to depose Clinton aides Abedin, Cheryl Mills, and Bryan Pagliano over the next two months.

The judge ruled that "it may be necessary" for the watchdog group to question Clinton herself, but it will need court permission to proceed.

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