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 OMB employee to testify in the House impeachment probe, appeared to confirm Democrats' assertion that the decision to withhold nearly $400 million in congressionally approved money for Ukraine, including millions in lethal aid, was a political one.

Sandy, OMB's deputy associate director for national security programs, 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. 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 Donald 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 White House orders. 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, who added that he did not raise a fuss because he wanted to keep his job.

His testimony does not appear to shed 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 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.

House Intelligence Committee Chairman Adam Schiff 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 re-election campaign."