The appearance by Volker is the first in what is expected to be a series of interviews with officials inside and outside the State Department.
Volker's meeting with the committee comes after an emotional day in the impeachment inquiry on Wednesday, when Trump railed at journalists during a news conference, resorted to the use of an expletive on Twitter and called the probe "a hoax and a fraud" while pledging to cooperate.
In a rough transcript of a July 25 telephone call between Trump and Zelenskiy, Trump asks as a "favor" from the Ukrainian president that he undertake an investigation of Biden.
The daylong session left uncertainties.
Trump asked Zelensky in that call to investigate former Vice President Joe Biden, a leading rival in Trump's bid for re-election in 2020.
Vitali Kasko, Ryaboshapha's deputy, told USA TODAY after the news conference that an affidavit by Shokin highlighted in recent days by Giuliani's office that claims he was sacked in 2016 because he was leading a "wide-ranging corruption investigation" into Burisma had no merit. The former prosecutor-general reportedly had been in contact with Giuliani, the person said.
In an exchange on August 29, Yermak texts Volker, "Need to talk with you", and then sends a link to a Politico story headlined "Trump holds up Ukraine military aid meant to confront Russian Federation". Aides to Pence dispute this assertion made by the Associated Press' sources, saying Biden never came up when Zelensky and Pence conversed. A 2005 law passed by Congress exempted immigrants from the broad DNA collection to which serious criminals are subjected, but DHS officials now say this exemption is outdated and ought to be eliminated.
A former State Department official said last week that "the fact that agents of the President, whether they be his son or Giuliani, should somehow be dragged into this mess on behalf of people who are unworthy and dubious at best is really distressing".
"This hearing is critical to establish additional details, leads and evidence", the Democrat who leads the committee, Rep. Adam Schiff, wrote last week about the hearing.
"If they were honest about it, they'd start a major investigation into Bidens", Trump said. "If a foreign country can help us with respect to corruption and corruption probes, I don't care if it's Biden or anybody else".
White House Press Secretary Stephanie Grisham and her top deputy, J. Hogan Gidley, have not responded to an email seeking comment on the contents of the text messages. In the complaint, the whistleblower says he or she had learned from US officials that "on or around 14 May, the President instructed Vice President Pence to cancel his planned travel to Ukraine to attend President Zelenskiy's inauguration on 20 May" and sent Energy Secretary Rick Perry to lead the delegation instead.