Washington (CNN) A federal judge Thursday called former Secretary of State Hillary Clinton's use of a private email server for government business "one of the gravest modern offenses to government transparency."

Judge Royce Lamberth blasted the Justice and State Departments for responses to a Freedom of Information Act lawsuit dating back to 2014 for Clinton's Benghazi-related emails.

"At best, State's efforts to pass off its deficient search as legally adequate during settlement negotiations was negligence born out of incompetence," Lamberth wrote. "At worst, career employees at the State and Justice Departments colluded to scuttle public scrutiny of Clinton, skirt FOIA, and hoodwink this court."

In a 10-page opinion, Lamberth ordered the government and Judicial Watch, a conservative watchdog group which brought the lawsuit, to come up with a discovery plan in 10 days.

Lamberth said the plan should examine if Clinton used a private email "to evade FOIA," if the State Department attempted to settle the case "despite knowing the inadequacies of the initial search constituted bad faith," and if State's searches for relevant records "have been adequate."

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