During her four years in President Barack Obama's Cabinet, Clinton relied on a private email account and server for both her work-related and personal messages. The practice led to a storm of controversy that roiled her 2016 presidential bid and is widely viewed as contributing to her ultimate defeat by Donald Trump.

In response to press questions during the campaign and in the sworn statement, Clinton said she kept the private account and server after taking over as secretary of state in 2009 as a matter of convenience and not to avoid FOIA or other disclosure requirements. The FBI investigated, interviewed Clinton and recommended against criminal charges, but it did find dozens of messages in her account that officials said contained highly classified information.

However, Lamberth said in his ruling Monday that the FBI probe and representations by the State Department have not adequately put to rest questions about the episode and Clinton's deposition is needed to address those concerns.

"To argue that the Court now has enough information to determine whether State conducted an adequate search is preposterous," wrote Lamberth, who has tangled with Clinton aides for decades in a series of cases.

The judge, an appointee of President Ronald Reagan, said the fact that more Clinton emails continue to dribble out from the State Department raises questions about the thoroughness of the government's earlier actions to recover Clinton's messages.

"Even years after the FBI investigation, the slow trickle of new emails has yet to be explained," Lamberth wrote. He blasted Clinton's prior answers given in response to another judge's order as "incomplete, unhelpful, or cursory, at best."

Among the questions Lamberth said Clinton still needs to answer: "How did she arrive at her belief that her private server emails would be preserved by normal State Department processes for email retention? Who told her that—if anyone—and when? Did she realize State was giving 'no records' response to FOIA requests for her emails? If so, did she suspect that she had any obligation to disclose the existence of her private server to those at State handling the FOIA requests? ... And why did she think that using a private server to conduct State Department business was permissible under the law in the first place?"

In addition to ordering Clinton's deposition, Lamberth said he'll require Clinton's former chief of staff Cheryl Mills to be deposed, although she was already questioned, in-person and under-oath by Judicial Watch about the issue in another FOIA case. The judge also approved a subpoena to Google for any Clinton emails it may possess.

Lamberth said Clinton and Mills may be questioned specifically about their knowledge of records related to the Benghazi attack, but not about their response to the assault, which killed four Americans including the U.S. ambassador to Libya.

Lawyers for Clinton and Mills did not immediately respond to requests for comment on the judge's order.