US ambassador Gordon Sondland has told congressional impeachment investigators that he worked with Rudy Giuliani on Ukraine at the "express direction" of Donald Trump and pushed a "quid pro quo" with Kiev because it was what the president wanted.
"Mr Giuliani was expressing the desires of the president of the United States, and we knew that these investigations were important to the president," Mr Sondland said of his dealings with Mr Trump's personal lawyer.
Mr Sondland, the most highly anticipated witness in the House of Representatives impeachment probe, made clear that he believed Mr Trump was pursuing his desire for investigations in return for the Oval Office meeting that the eastern European nation's president sought.
The witness said he later came to believe military aid for Ukraine was also being held up until the investigations were launched.
He described how demands became more serious with more conditions on any potential Ukraine meeting at the White House.
"As time went on, more specific items got added to the menu – specially Burisma and 2016 meddling," he said, referring to the gas company where Joe Biden's son Hunter served on the board. And, he added, "the server", the hacked Democratic computer system.
"I know that members of this committee have frequently framed these complicated issues in the form of a simple question: Was there a 'quid pro quo?' As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes," he said.
Mr Sondland said he did not know at the time that Burisma was linked to the Bidens, but has since come to understand that.
"We had been hearing about it from Rudy and presumed Rudy was getting it from the president."
The impeachment inquiry focuses significantly on allegations that Mr Trump sought investigations of Democrat Joe Biden and his son - and the discredited idea that Ukraine rather than Russia interfered in the 2016 US election - in return for military aid as well as the White House visit.
Mr Sondland, a wealthy hotelier and Trump donor, has emerged as a central figure in an intense week in the impeachment probe that has featured nine witnesses giving evidence over three days. Democrats and Republicans were uncertain about what he would testify to, given that he had already clarified parts of his initial private deposition before legislators.
His opening statement included several key details. He confirmed that he spoke with Mr Trump on a mobile phone from a busy Kiev restaurant the day after the president prodded Ukraine's leader to investigate Mr Biden.
He also said he kept secretary of state Mike Pompeo and other senior administration officials aware of his dealings with Ukraine on the investigations Mr Trump sought.
Mr Sondland said he specifically told vice president Mike Pence he "had concerns" that US military aid to Ukraine "had become tied" to the investigations.
"Everyone was in the loop," he said in opening remarks. "It was no secret."
He insisted he was "adamantly opposed to any suspension of aid" for Ukraine. "I was acting in good faith. As a presidential appointee, I followed the directions of the president."