Sondland's disclosure appeared meant to insulate him from accusations that he intentionally misled Congress during his earlier testimony, in which he frequently said he could not recall key details and events under scrutiny by impeachment investigators.
Democrats had said they had enough material to move forward with public impeachment hearings, which would be a likely prelude to articles of impeachment - formal charges - against Trump being brought to a vote in the House.
An anonymous whistleblower complaint in September highlighted potential abuse of power by the president when he telephoned Ukraine's leader and asked him to investigate former vice president Joe Biden, Trump's potential 2020 election rival.
"That was my clear understanding, security assistance money would not come until the president (Zelenskiy) committed to pursue the investigation", Taylor said.
Kent, who heads the European and Eurasian bureau at the State Department, expressed concerns about White House efforts to remove the then-ambassador to Ukraine, Marie Yovanovitch.
Three other witnesses summoned by the House committees were not expected to appear.
According to the AP, Hale is expected to say that Pompeo and the state department did not protect Yovanovitch for fear of angering Giuliani, a private citizen who had conducted a lobbying campaign against the ambassador.
He was similarly disturbed by a subsequent phone call that day with Sondland, who "told me that President Trump had told him that he wants President Zelensky to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 US election", he said. That claim of a quid pro quo initially contradicted Sondland's testimony, but this week, Sondland said he misremembered events, and confirmed the accuracy of Taylor's account.
Mr. Kent said he was alarmed by the effort to get Ukraine to investigate Mr. Biden and his son Hunter, and the 2016 election.
"I think the origin of the idea to get President Zelenskiy to say out loud he's going to investigate Burisma and 2016 election, I think the originator-the person who came up with that-was Mr. Giuliani", Taylor said.
The Democrats are investigating Mr Trump's requests for allegedly politically motivated probes by Ukraine as the USA withheld military aid from the country.
The elder Biden is a leading candidate in the race for the Democratic nomination to challenge Trump for the presidency in 2020.
13, the public will be able to hear directly from witnesses who have already been interviewed behind closed doors, according to Politico.
But despite those denials, Schiff said Wednesday that the witnesses will show that "the most important facts are largely not contested" in the inquiry.
Congressional Democrats also released testimony from Mr Kurt Volker, Mr Trump's former special representative for Ukraine negotiations. There was no quid pro quo. In the revision, Sondland said his memory had been "refreshed" and that he did recall telling Yermak about the link between the aid and the investigation sought by the White House.
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