On Tuesday, he said Gordon Sondland, the USA ambassador to the European Union, made a "generic" comment about investigations that everyone regarded as inappropriate. "I didn't think there was a pay-up", Volker said on November 19, adding that he only knew the "generic formulation" that Trump had a negative view of Ukraine and wanted assurances before he paid them the "check" with USA aid.
Volker testified in Congress on Tuesday as a part of the impeachment hearings before the House Intelligence Committee.
During the call, Mr Trump asked Mr Zelenskiy to carry out two investigations that would benefit him politically, including one targeting Democratic political rival Joe Biden.
Tuesday's morning session will also feature Jennifer Williams, an aide to Vice President Mike Pence, who was among the US officials who monitored the July 25 Trump-Zelensky call and testified behind closed doors this month that some of Trump's comments were "inappropriate".
Two other witnesses in a second session, former United States special envoy to Ukraine Kurt Volker and National Security Council aide Tim Morrison, supported allegations the White House sought the political assistance.
In closed-door testimony in October, Volker said he had not heard any references to investigations during a July 10 White House meeting between USA and Ukrainian officials.
The White House published tweets questioning Vindman's judgment - even as he continues to serve on the president's National Security Council.
University of Calgary encourages its exchange students to vacate Hong Kong
The protesters, meanwhile, made gains on a legal front when the high court struck down the ban on face masks. The city's hospital authority reported 221 injuries over the past day, with 12 people in serious condition.
Then, when people like Volker, Morrison and Lt. Col. Alexander Vindman testify about President Trump's call with Ukranian President Volodymyr Zelensky later on, "all of the big dogs say it wasn't a problem". "I never saw the man", Trump said.
Vindman is the top Ukraine expert on the national security council and previously told impeachment investigators behind closed doors that Donald Trump's call with the Ukrainian president so alarmed him that he immediately informed White House lawyers.
The investigation could lead the House to approve formal charges - known as articles of impeachment - against Trump that would be sent to the Republican-controlled Senate for a trial that could result in his removal from office. He told lawmakers that "character attacks" against public servants testifying in the impeachment inquiry were "reprehensible".
The whistleblower's identity has remained a secret, but Mr Trump and his allies have repeatedly called for the individual to be identified.
"Do not worry, I will be fine for telling the truth", he said.
"It's the kind of thing you say when you're defending the indefensible", House Democrat Jim Himes said.
Nine officials in total are set to appear this week, including, on Wednesday, Sondland, who other witnesses indicated had direct communications with Trump about pressuring Ukraine.