An FBI official issued a rare acknowledgement of the bureau's investigation into Hillary Clinton's private server use Monday in a letter filed in federal court.

Although FBI Director James Comey confirmed the existence of the probe in October of last year, agency officials have repeatedly declined to comment on the nature of the investigation and the direction in which it is headed.

James Baker, general counsel for the FBI, wrote in a letter filed in a Freedom of Information Act lawsuit that the FBI "has acknowledged generally that it is working on matters related to Secretary Clinton's use of a private email server."

Baker declined to elaborate on the "specific focus, scope or potential targets" of the investigation, arguing the probe could be compromised if he did so.

Baker also said "intervening events" have changed the FBI's initial response to the court from September, but noted he was unable to comment further.

The FOIA lawsuit in question was filed by conservative watchdog Judicial Watch, which is presently locked in a legal battle with the State Department over the records of Clinton and her aides.

Judicial Watch has argued the records on Clinton's private server, which is in FBI custody, should be subject to review under FOIA.