A federal judge on Monday ordered former Secretary of State Hillary ClintonHillary Diane Rodham ClintonFormer Obama adviser Plouffe predicts 'historical level' of turnout by Trump supporters Poll: More Republican voters think party is more united than Democratic voters Whoopi Goldberg presses Sanders: 'Why are you still in the race?' MORE to give a deposition under oath over a years-old lawsuit filed by the conservative group Judicial Watch regarding her role in the response to the 2012 attack on the U.S. compound in Benghazi, Libya.

U.S. District Court Royce Lamberth, a Reagan appointee, said in court filings that written answers provided by Clinton to questions from Judicial Watch were "incomplete, unhelpful, or cursory, at best."

"As extensive as the existing record is, it does not sufficiently explain Secretary Clinton’s state of mind when she decided it would be an acceptable practice to set up and use a private server to conduct State Department business," Lamberth wrote in his order. "Simply put, her responses left many more questions than answers."

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"Even years after the FBI investigation, the slow trickle of new emails has yet to be explained," the judge added, referring to the rate at which emails from Clinton's private email server have since been released.

The order is the first requiring Clinton to testify under oath in person on the subject of the 2012 attack on the U.S. compound in Benghazi and the Obama administration's response at the time, when Clinton was head of the State Department.

Conservatives have long been critical of Clinton's role in the administration's response to the attack, which resulted in the death of several Americans including the U.S. ambassador to Libya, Christopher Stevens.