The FBI confirmed in a letter that it is investigating Hillary Clinton’s use of a personal, unsecured email server during her time as secretary of state.

MSNBC reported that the FBI’s general counsel James Baker wrote in a letter dated February 2 that the government agency "has acknowledged generally that it is working on matters related to former Secretary Clinton’s use of a private email server" in public statements and testimony to Congress.

The letter, dated February 2, was disclosed in a federal court filing Monday. Baker further wrote that the FBI has not "publicly acknowledged the specific focus, scope or potential targets of any such proceedings." He concluded the letter by saying that the FBI cannot disclose more "without adversely affecting on-going law enforcement efforts."

The letter was filed in a Freedom of Information Act (FOIA) lawsuit brought against the State Department by government watchdog group Judicial Watch.

The formal confirmation from the FBI comes just one day before Clinton and her competitor for the Democratic nomination, Sen. Bernie Sanders (I., Vt.), face off in the New Hampshire primary. Polls have consistently showed Sanders leading Clinton by a wide margin in the state.

Clinton’s poll numbers and favorability rating have declined in the wake of revelations about her use of personal email.

Clinton has endured scrutiny for months for her use of private email to conduct sensitive government business. It has been widely reported since last August that the FBI was probing Clinton’s private email setup, though the agency had yet to confirm it.

The Obama administration confirmed last month that 22 emails on Clinton’s personal server contain top secret information, and the State Department withheld the messages from public release.

Still, Clinton has insisted that she never knowingly sent nor received information marked classified on her personal email.