(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.

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