While the State Department and National Archives took steps to recover the emails from Clinton's tenure, they did not ask the U.S. attorney general to take enforcement action. Two conservative groups filed lawsuits to force their hand.

A district judge in January ruled the suits brought by Judicial Watch and Cause of Action moot, saying State and the National Archives made a "sustained effort" to recover and preserve Clinton's records.

But Williams said the two agencies should have done more, according to the ruling in the U.S. Court of Appeals for the District of Columbia Circuit. Since the agencies neither asked the attorney general for help nor showed such enforcement action could not uncover new emails, the case was not moot.

"The Department has not explained why shaking the tree harder - e.g., by following the statutory mandate to seek action by the Attorney General - might not bear more still," Williams wrote. "Absent a showing that the requested enforcement action could not shake loose a few more emails, the case is not moot."