U.S. District Judge Royce C. Lamberth ordered the hearing after a legal watchdog asked the court to obtain testimony under oath from current and former State Department officials, including Clinton and former aide Cheryl Mills.

If Lamberth agrees with Judicial Watch, Clinton and others would have to testify under oath and answer questions about how the department processed the organization's Freedom of Information Act requests, and how it conducted its search for emails Clinton sent and received over a private email server.

Mills was Clinton's chief of staff and counselor when she was secretary of state from Jan. 21, 2009, to Jan. 31, 2013, and worked on her presidential campaigns.

The State Department had asked Lamberth to issue a summary judgment that would have closed the case and ended any more inquiries into Clinton emails that have not been disclosed.