The FBI interviewed Hillary Clinton's closest ally Huma Abedin on Thursday in the latest development of the federal government’s probe into her unsecured email system, according to U.S. officials.

CNN reported:

In recent weeks, multiple aides have been interviewed — some more than once, the officials said. A date for an FBI interview of Clinton has not been set, these officials said, but is expected in the coming weeks. Abedin has cooperated with the probe, the officials said.

While the investigation is ongoing, investigators said they still haven't discovered evidence that shows the Democratic presidential front-runner "willfully" violated federal law.

Officials are seeking to determine whether Clinton mishandled classified information by using a private email system during her time serving as secretary of state.

Though the probe is nearing completion, the case's final review threatens to dog Clinton for the remainder of the presidential primary elections.

Clinton has repeatedly brushed aside the probe as a nonissue on the campaign trail, arguing that none of the emails sent or received on her private server were marked classified at the time. The State Department meanwhile has upgraded thousands of messages to classified status throughout its review of Clinton’s work-related correspondences.

The Washington Post in March that roughly 50 FBI agents were investigating the case.

Government officials found that more than 2,000 emails on Clinton’s server contain classified information, though none was marked classified on her personal email. Twenty-two messages have been deemed "top secret," the highest classification level, and were withheld from public release