Washington (CNN) State Department officials cannot identify which staff members and officials may have used Hillary Clinton's private email server to conduct government business, government lawyers said in a new court filing released Thursday.

Judicial Watch, which is fighting the State Department in one of a series of lawsuits seeking information on how the Clinton team used email in the State Department, announced Thursday that State lawyers could not track which employees were conducting business on the Clinton email server.

Asked in a court interrogatory about who had used clintonemail.com addresses to conduct government business, State Department lawyers objected to the question by saying they had "no method of identifying" who was using the private email system.

"State objects to this interrogatory on the grounds that it never possessed or controlled clintonemail.com, does not now possess or control clintonemail.com, and thus has no method of identifying which State Department officials and employees had and/or used an account on clintonemail.com to conduct official government business," lawyers wrote in the filing, released by Judicial Watch Thursday.

Clinton herself and top aide Huma Abedin have both said they used the private email server -- but it's unclear if other top Clinton aides, including Jake Sullivan, also used clintonemail.com addresses.

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