Former U.S. Ambassador to Ukraine Marie Yovanovitch testifies before the House Intelligence Committee in the Longworth House Office Building on Capitol Hill November 15, 2019 in Washington, DC.
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(CNN) —  

House impeachment investigators have little visibility into the inner workings of the White House at the time when President Donald Trump allegedly ordered a hold on military assistance for Ukraine.

That could change on Saturday when Mark Sandy, a senior career official at the Office of Management and Budget, is expected to testify behind closed doors.

Sandy will become the first OMB official to meet with investigators after others have refused. Political appointees in the office have fought subpoenas, refused to provide documents, and tried in other ways to stonewall the probe.

He is senior enough that he led the agency as acting director in the early weeks of the Trump administration, before political appointees could be put in place. He previously worked under both Republican and Democrat administrations.

“He’s a longstanding public servant with an excellent reputation,” said Sam Berger, a high-ranking OMB political appointee from the Obama administration who worked with Sandy.

The impeachment investigation is focused on whether Trump and his allies withheld funding for Ukraine’s military, which is fighting a Russian invasion, while seeking a political favor from the country’s President.

From his perch in OMB’s national security programs office, Sandy is expected to have insight into internal conversations when the administration was taking the unusual step of freezing the funding. A source familiar with the matter said Sandy did not know at the time why it was frozen.

The funding was held earlier this year at the direction of Trump, according to an administration official and testimony of Catherine Croft, a diplomatic aide.

Michael Duffey, the Trump-appointed associate director overseeing the office Sandy works in, signed some of the documentation that froze the funding, according to two sources.

The freeze stood out for several reasons. The sources said it is highly unusual for political appointees to sign the documents holding the funding. But career officials had concerns that the hold could violate federal laws requiring the administration to spend money as directed by Congress, a congressional aide told CNN.

Croft said in her closed-door testimony that there were conversations in the administration about the legality of the hold.

“Under normal course of business, Sandy would be the person to sign off on this kind of action,” Berger said. “Making an end run around him means the Trump White House knew it was asking for something Sandy wouldn’t do.”

Duffey has not testified to House investigators, but a separate source familiar with the situation explained Duffey signed the freeze paperwork because he was new to the job and believed Trump, whom he thought is generally skeptical of foreign aid, would want to review it.

Sandy’s testimony could shed light on those behind-the-scenes conversations and help investigators determine how widespread was the knowledge of the President’s alleged purpose for the hold.

Unlike other White House officials whose testimony investigators have sought, Sandy has a low public profile.

Also unlike those other officials, Sandy will testify, according to his attorney. He did not appear for a previous testimony date but is expected to receive a subpoena, as the committee has done for other career government officials, giving them cover to testify.

Mick Mulvaney, who is both the OMB director and President’s acting chief of staff, initially sued to block a subpoena for his testimony and has not spoken with investigators. Russ Vought referred to the probe as a “sham process” and did not appear to testify. OMB has declined to turn over documents to investigators.

CNN’s Manu Raju and Jeremy Herb contributed to this report.