The top aide to Secretary of State Hillary Clinton was told in late 2012 that a non-profit watchdog group had requested information about Clinton’s email, including the number of accounts she used, according to documents released Wednesday.

The aide, Chief of Staff Cheryl Mills, and other Clinton advisers knew at the time that the secretary was using a private email system to conduct official business. But despite the information request, the State Department told the group that it had found no “responsive” records.

“This is evidence that Cheryl Mills covered up Hillary Clinton’s email system,” Tom Fitton, the president of Judicial Watch, a conservative group that sued for information about that original 2012 request, said in a statement. “She was aware of the [Freedom of Information Act] request about Clinton’s email accounts and allowed a response to go out that was a plain lie. And you can bet if Cheryl Mills knew about this inquiry, then Hillary Clinton did, too.”

A Clinton campaign spokesperson didn’t immediately respond to a request for comment.

Mills was alerted to the request, from the group Citizens for Responsibility and Ethics in Washington (CREW), via an email from a State Department spokesman, Brock Johnson, on Dec. 11, 2012. Mills was told that the group had filed a “significant” request seeking “records sufficient to show the number of email accounts of or associated with Secretary Hillary Rodham Clinton…”

More than two years later, the fact that Clinton had been using a private account during her tenure finally came to light. House investigators looking into the terrorist attacks on a U.S. diplomatic post in Benghazi, Libya, discovered Clinton emails using a .com account. The existence of the private email setup was first reported by The New York Times.

CREW had looked into Clinton’s email setup after revelations that Lisa Jackson, the administrator of the Environmental Protection Agency, had been using a private email account.

The State Department inspector general had previously reported on CREW’s request and the department’s failure to find any records, which the IG called an “inaccurate and incomplete” response. But the email to Mills, whom the IG did not identify by name, is the first public documentation on the matter and will provide Clinton’s critics with more evidence that she and her aides were not forthcoming.

The IG criticized officials for coming up with little or no response to CREW’s requests and those from other outside groups even though the information they sought was clearly available.

Judicial Watch has already deposed Mills and other top Clinton aides under oath in a lawsuit seeking information about how and why Clinton’s email system was set up. Now, Judicial Watch wants the judge in that case to force Clinton to give a deposition, as well.

“This is all the more reason for Mrs. Clinton to finally testify under oath about the key details of her email practices,” Fitton said, pointing to the evidence that Mills was aware of inquiries on the subject while Clinton was still in office.