WASHINGTON — The first official from inside the White House Office of Management and Budget to break ranks and testify in the impeachment inquiry against President Donald Trump appeared for his closed-door deposition Saturday morning, offering insight into the decision to delay military aid to Ukraine.

House investigators have spoken to officials from the Pentagon and the State Department about the hold placed on the congressionally appropriated funds, but they're hoping Mark Sandy, a longtime career employee, will fill out their understanding of what transpired when he meets with them privately.

The allegation that Trump held up nearly $400 million in military and security aid to Ukraine, hoping to use it as leverage to pressure the foreign government to announce an investigation into former vice president Joe Biden and his son Hunter, is central to the Democrats' investigation.

Sandy arrived shortly before 10 a.m. for his sworn deposition.

Sandy’s name came up in the closed-door deposition of Laura Cooper, deputy assistant secretary of defense for Russia, Ukraine and Eurasia, according to a transcript of her testimony. Cooper, reading from “apportionment letters” — documents required to hold up the money to Ukraine — affirms that it’s Sandy’s signature on it, though she says she does not know who he is.

But then, in subsequent letters related to Ukraine funds, the job of signing off on them was given to a political appointee, Michael Duffey, OMB's associate director for national security programs.

Cooper testified that Duffey, in a meeting on July 26 — a day after Trump’s call with Ukrainian President Volodymyr Zelensky — said the hold on the military aid “relates to the President’s concerns about corruption.”

House investigators will want to know from Sandy how and why that change was made. People familiar with the matter have said he was among the career officials at OMB who raised questions about why the money, already approved by Congress, was being delayed.

Sandy is one of four OMB employees called to testify, but the only one who isn't a political appointee. The others, who include Duffey and OMB acting director Russell Vought, defied congressional subpoenas, heeding a White House demand that administration officials not participate in the impeachment investigation.

Sandy has worked at the agency on and off for over a decade, under presidents of both parties, climbing the ranks to his current role as deputy associate director for national security programs.

Sandy’s attorney did not return request for comment.