Washington — Longtime White House budget employee Mark Sandy appeared in a closed hearing on Saturday before the House committees conducting the impeachment inquiry, making him the first official from the Office of Management and Budget (OMB) to testify in the inquiry.

Sandy's testimony may have filled in gaps about the withholding of military aid to Ukraine for several months, a process in which the OMB played a critical role. Unlike several other OMB officials who have refused to appear before the committee, he was not appointed by President Trump.

The Washington Post first reported a statement by Sandy's lawyer that he would appear if subpoenaed, in a break with other White House officials. Sandy's lawyer confirmed to CBS News on Thursday that Sandy would appear if subpoenaed, and on Saturday morning he was photographed arriving for his deposition.

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An official working on the impeachment inquiry told CBS News that Sandy was appearing under subpoena.

"In light of an attempt by OMB to direct Mark Sandy not to appear for his scheduled deposition, and efforts to limit any testimony that does occur, the House Intelligence Committee issued a subpoena to compel his testimony. As required of him by law, Mr. Sandy is complying with the subpoena and answering questions from both Democratic and Republican Members and staff," the official said.

Mark Sandy arrives to the U.S. Capitol for a closed door deposition with lawmakers regarding whether President Trump ordered a hold on military assistance to Ukraine, November 16, 2019. Getty Images

White House counsel Pat Cipollone informed Congress in September that the White House would not cooperate with the impeachment inquiry, and the Office of Management and Budget has declined to participate.

Russ Vought, the acting director of the OMB, tweeted in October that he and Mike Duffey, another top OMB official, would not comply with congressional requests for depositions. "As the WH letter made clear two weeks ago, OMB officials - myself and Mike Duffey - will not be complying with deposition requests this week. #shamprocess," Vought wrote on Twitter on October 21.

White House chief of staff Mick Mulvaney has also defied a subpoena to appear before the committees conducting the impeachment inquiry.

In a statement released after Sandy's testimony, House Intelligence Committee Chairman Adam Schiff condemned Mulvaney for defying the subpoena.

"If Mulvaney had evidence that contradicted what we've already heard, he'd be eager to testify and provide documents. Instead, he's hiding behind, and assisting in, Trump's efforts to conceal the truth from the American people," Schiff said.

Sandy's testimony follows the first two open hearings in the impeachment inquiry this week, as well as a closed hearing with State Department staffer David Holmes, the aide who overheard a phone call between U.S. Ambassador to the EU Gordon Sondland and President Trump.