A top official at the Office of Management and Budget indicated Thursday he is willing to testify in the House impeachment inquiry, his attorney said.

Mark Sandy would be the first employee from the OMB to defy the White House and appear before Congressional investigators. The White House has urged administration officials not to comply with what they are calling a sham investigation.

“If Mr. Sandy is subpoenaed, he will testify this Saturday,” Barbara Van Gelder, an attorney at Cozen O'Conner, said in an email.

The news was first reported by the Washington Post.

Sandy, an associate director for National Security Programs, has been asked to appear Saturday for a deposition by the House Intelligence, Foreign Affairs and Oversight and Reform Committees. A spokesman for the House Intelligence Committee declined to comment Thursday night.

Sandy is considered a critical witness who can provide insight into the withheld security aid to Ukraine, which is at the basis of the impeachment inquiry opened by House Democrats.

He is “a true public servant who has served with great distinction in both Republican and and Democratic administrations,” said Nicholas Rasmussen, former director of the National Counterterrorism Center under the Barack Obama and Trump administrations.

Democrats are interested in hearing from Sandy and other OMB career officials about the agency’s role in withholding nearly $400 million in security aid to Ukraine and the eventual release of that aid Sept. 11 after the hold on the aid became public, causing a Congressional uproar and prompting the committees to open an investigation.

Multiple people have testified that they learned of the hold on the aid from the OMB.

Defense Department official Laura Cooper testified that President Donald Trump’s acting Chief of Staff Mick Mulvaney “conveyed” the message that the aid was withhold at Trump's direction in a mid-July meeting.

Bill Taylor,acting U.S. ambassador to Ukraine, said he learned the aid was being withheld in a conference call with OMB, in which an official said it was directed by the president.

Three other career OMB officials have defied Congressional subpoenas and refused to appear before the committees, including Russel Vought, acting director; Michael Duffey, an associate director for National Security Programs; and Brian McCormack, an associate director for natural resources, energy and science.