Donald Trump’s acting chief of staff Mick Mulvaney approved a White House meeting with the president for the Ukrainian president on condition Ukraine announced investigations tied to Trump’s political rival Joe Biden, according to testimony released on Friday.

Gordon Sondland, the ambassador to the European Union, “blurted out” that Mulvaney had approved the meeting if the Ukrainians announced an investigation of Burisma, a gas company that formerly employed Hunter Biden, the former vice president’s son, said Fiona Hill, a national security council member who was deposed last month by the congressional committees pursuing an impeachment inquiry against Trump.

Hill’s account was corroborated by simultaneously released testimony by another firsthand witness to the conversation, Lt Col Alexander Vindman.

Previously released testimony has indicated a central role for Mulvaney in brokering an agreement in which Ukraine would intervene in the 2020 US election by announcing the Burisma investigation, but the Hill testimony released on Friday was the first to describe direct involvement in the plot by the acting chief of staff.

Mulvaney himself said in a televised October press conference that the White House had conditioned military aid on investigations in Ukraine, but he later denied he had said that.

Hill told investigators of a 10 July meeting attended by herself, Sondland, nationals security adviser John Bolton, Ukrainian officials and others.

Hill testified: “Then Ambassador Sondland blurted out: ‘Well, we have an agreement with the chief of staff for a meeting if these investigations in the energy sector start.’ And Ambassador Bolton immediately stiffened and ended the meeting.”

Vindman gave a similar account of the meeting.

“I heard him say that this had been coordinated with White House chief of staff, Mr Mick Mulvaney,” Vindman said. “He just said that he had had a conversation with Mr Mulvaney, and this is what was required in order to get a meeting.”

Mulvaney was further responsible for ordering a hold on almost $400m in military aid for Ukraine, Vindman testified. He explained the reason was “to ensure that the assistance aligned with administration priorities”.

With eight transcript releases over the past week, the impeachment committees have revealed a lot of mutually reinforcing evidence that the Trump administration tried to force Ukraine into domestic US politics, while taking steps to stop the plot from becoming public, alarming dozens of top-level US officials, who are defying a White House gag order to explain what happened.

While Trump himself made the most direct known ask of the Ukrainian president, Volodymyr Zelenskiy, in a 25 July phone call previously summarized by the White House, the day-to-day management of the plot largely fell to Mulvaney and to Rudy Giuliani, according to testimony.

Testimony released Friday also shed new light on White House efforts to cover up its negotiations with Ukraine; on the destructive impact the plot had on the morale of career public servants; and on the borderline slapstick role of Sondland, a hotelier and Trump mega-donor with no previous diplomatic experience.

Sondman pursued a Ukrainian delegation visiting the White House in July out of the meeting that Bolton ended into a second meeting in the downstairs Ward Room, attended by Vindman.

“What did you hear Sondland say?” Vindman was asked during his testimony.

“That the Ukrainians would have to deliver an investigation into the Bidens,” Vindman replied.

“Into the Bidens. So in the Ward Room he mentioned the word ‘Bidens’?”

“To the best of my recollection, yes.”

On Friday, Trump distanced himself from Sondland, who gave his inauguration committee $1m, telling reporters: “I hardly know the gentleman.”

Hill and Vindman were startled by the nakedness of the deal Trump was chasing in the July phone call with Zelenskiy, which both listened in on, they testified. “It was pretty blatant,” Hill said.

Askedif there was “any doubt in your mind as to what the president, our president, was asking for as a deliverable”, Vindman replied: “There was no doubt.”

When he raised his concerns about the call with national security council lawyers afterward, Vindman said, he never heard any follow up. And the call was placed, abnormally, in a separate, highly restricted classification system, which became an issue when Vindman went to add his edits to the summary, per normal operating procedure.

The call summary released by the White House included unusual omissions and corrections, including the deletion of a mention by Zelenskiy of “Burisma”, which was replaced in the summary with “the company”.

The change was significant, Vindman said, because it hides the fact that when Zelenskiy heard Trump say “Biden”, he knew exactly what he was talking about – an investigation of Burisma.

“It would be significant,” Vindman said. “Because – because, frankly, the President of Ukraine would not necessarily know anything about this company Burisma. I mean, he would certainly understand some of this – some of these elements because the story had been developing for some time, but the fact that he mentioned specifically Burisma seemed to suggest to me that he was prepped for this call.”