Washington: House impeachment investigators met for a rare weekend session on Saturday to privately question a senior official from the White House budget office about President Donald Trump’s decision this summer to freeze $391 million (Dh1.28 billion) in security assistance to Ukraine.

Why precisely Trump withheld the congressionally allocated funding in mid-July as he pressed Ukraine for politically beneficial investigations and what his acting chief of staff, Mick Mulvaney, told the agency about the decision remain central unanswered questions in the inquiry.

Democrats leading the proceedings had hoped that the budget official, Mark Sandy, can at least offer a glimpse into deliberations at the Office of Management and Budget over carrying out the order.

In the end, Sandy told investigators that he did not know why the military assistance had been delayed but that he had never encountered a similar situation in his time at the agency, according to two people familiar with his testimony.

As soon as the interview with Sandy concluded, Democrats released transcripts of two more witness interviews that took place in recent weeks. They included Timothy Morrison, the senior director for Europe and Russia for the National Security Council, and Jennifer Williams, a longtime State Department employee with expertise in Europe and Russia who is detailed to Vice President Mike Pence’s national security staff.

Morrison confirmed to investigators two key episodes at the centre of the inquiry that suggest at least some high-ranking government officials believed Trump was conditioning the release of military aid on Ukraine’s public commitment to the investigations he sought and informed Ukrainians officials that was the case.

Both Morrison and Williams listened in on a July 25 phone call in which Trump pressed Ukraine’s new president to investigate former Vice President Joe Biden and his younger son, Hunter, as well as a debunked conspiracy theory about Ukraine and the 2016 election.

Other witnesses with significant roles in American diplomacy toward Ukraine, including some working closely with Trump, have already testified that the aid was delayed as part of a broad pressure campaign meant to extract a public commitment from Ukraine to investigate the president’s political rivals.

Democrats have marshaled that testimony to begin arguing that Trump may have committed bribery to get what he wanted from Ukraine. But they have yet to hear from a witness who can speak directly to the president’s order and stated rationale.

Sandy testified after the House Intelligence Committee subpoenaed him Saturday morning, a day after the former American ambassador to Ukraine, Marie Yovanovitch, described in stark and personal terms how the president and his allies sought to undermine her and push her out of her job. The budget office had directed him not to appear, according to an official working on the inquiry.

Trump continued his scorched-earth defence on Saturday, denouncing those involved in the proceedings. In one tweet, he misspelled the name of the chairman of the House Intelligence Committee leading the inquiry, Rep Adam Schiff, D-Calif, to what sounds like a vulgarity, claiming the stock markets would collapse if he were impeached. And the president, attributing a quotation to the conservative radio host Rush Limbaugh, suggested that non-partisan diplomats who have testified were aggrieved members of the Washington “Swamp” merely trying to exact their revenge.

“It is paramountly obvious watching this, these people have to go,” Limbaugh said, according to the president.

On Friday, Trump targeted Yovanovitch on Twitter as she was testifying, prompting heavy criticism, including from Democrats who accused him of witness intimidation.

Sandy is the first budget official to speak with impeachment investigators, in defiance of a Trump administration directive not to cooperate.

At least three higher-profile Trump administration officials connected to the budget office have defied investigators: Russell T Vought, the agency’s acting director; Michael Duffey, who helped carry out Trump’s directive to freeze the aid; and Mulvaney, who retains the title of budget director.

A Defence Department official has already testified that she and others involved in Ukraine policy raised concerns about whether the hold would present legal problems if it was not reversed. Other witnesses have described how the president’s most senior advisers, including the secretaries of state and defence, pushed him to unfreeze the aid in August.

Late Friday night, investigators interviewed David Holmes, an official from the United States Embassy in Kyiv, who described a call he overheard in July in which Trump asked his ambassador to the European Union whether the Ukrainian president had committed to an investigation into a leading political rival that Trump had personally pressed him to conduct a day earlier.

As of Saturday morning, House Democrats, who control the inquiry, had not scheduled any additional private witness interviews. But there will be three days of public hearings in the coming week, featuring some of the biggest names ensnared in the inquiry.