A longtime budget official testified Saturday that the White House decision to freeze military aid to Ukraine in mid-July was highly irregular and that senior political appointees in the Office of Management and Budget were unable to provide an explanation for the delay.

The testimony from Mark Sandy, the first employee of OMB to testify in the House impeachment probe, appeared to confirm Democrats’ assertion that the decision to withhold nearly $400 million in congressionally approved funds for Ukraine, including millions in lethal aid, was a political one.

Sandy, the deputy associate director for national security programs at OMB, testified that he was instructed to sign the first of several apportionment letters in which budget officials formally instituted the freeze on funds, according to two people familiar with his testimony who spoke on the condition of anonymity to speak frankly. He was never given a specific reason as to why the letter was being sent out, the people added.

Other witnesses have testified that the letter Sandy signed was dated July 25 — the same day that President Trump spoke by phone with Ukrainian President Volodymyr Zelensky, and one week after OMB verbally informed interagency officials that they were withholding the funds on orders from the White House. The signature of Sandy’s boss, political appointee Michael Duffey, appears on subsequent letters.

Sandy testified that the change came about when Duffey told him he wanted to learn more about the budget apportionment process. Sandy thought this was odd, the people familiar with his testimony said — and he told investigators that he suggested to Duffey that if he wanted to learn more about the process, there were other ways to do it. But Duffey insisted.

Sandy testified that he had never in his career seen a senior political OMB official assume control of a portfolio in such a fashion, according to the people familiar with his testimony.

His testimony does not appear to shed any new light on whether the Trump administration’s decision to withhold Ukraine aid was leverage to pressure that country’s government to launch investigations against Trump’s political rivals. But it undermines acting White House chief of staff Mick Mulvaney’s public assertion that the Ukraine aid was frozen in a routine manner that happened “all the time.”

Rep. Jamie B. Raskin (D-Md.), one of the lawmakers in the closed-door deposition, told reporters Saturday that Sandy’s testimony represented “a technical part of our investigation.”

“We want to know exactly how the president translated his political objective to shake down the Ukrainian government for the favors he wanted, translated into the budget process,” he said. But he hinted that more testimony from other OMB witnesses might be necessary to round out that story.

“Everybody owes Congress their honest and true testimony, and people should come forward,” he said.

House Intelligence Committee Chairman Rep. Adam B. Schiff (D-Calif.) singled out Mulvaney for refusing to testify in the House’s probe, even though he “publicly admitted from the White House podium that Trump withheld the military aid in order to pressure Ukraine to conduct investigations meant to help the president’s reelection campaign.”

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

Thus far, the panels have heard from 17 witnesses, 11 of whom have either given or are scheduled to offer public testimony. Those witnesses span the State Department, Pentagon, National Security Council and the Office of the Vice President. Sandy is the first OMB official to testify, which he did for almost six hours under subpoena, according to an official working on the impeachment inquiry. OMB had instructed him not to cooperate.

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 is Sandy’s signature on it, though she says she does not know who he is.

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

Sandy is one of four OMB employees called to testify, but he is the only one who is not a political appointee. The others, who include Duffey and OMB acting director Russell T. 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.

CORRECTION: An earlier version of this story said Mark Sandy, a career OMB official, did not object to a political appointee taking over decision-making on aid to Ukraine because he wanted to keep his job, according to two people familiar with his closed-door deposition Saturday. Sandy’s lawyer, Barbara Van Gelder, said in a statement Sunday that her client did not express fear about losing his job. This article has been updated to reflect that.