William Taylor, the top U.S. diplomat to Ukraine, testified to Congress on Wednesday, expressing concerns about the U.S. withholding military assistance to Ukraine last summer as well as President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE pressing the Ukrainian president for "investigations."

Trump's interaction with Ukraine is at the heart of House Democrats' impeachment inquiry, which was prompted in part by a July 25 call between Trump and Ukraine's president in which Trump urged Volodymyr Zelensky to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE's son and his role in the Ukrainian company Burisma.

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Taylor, in previous closed-door testimony before the House, was the first official to testify that he believed quid pro quo existed between Trump's request for investigations and withholding Ukrainian military aid.

In his opening statement on Wednesday, taking place in a public hearing, he also described for the first time a conversation between a member of his staff and U.S. Ambassador to the European Union Gordon Sondland that followed a phone call between Sondland and Trump last July. Sondland reportedly told that staffer, who inquired about Trump's thoughts on Ukraine after the call, that "President Trump cares more about the investigations of Biden, which [Trump's personal attorney Rudy] Giuliani was pressing for."

"I had come to understand well before then that 'investigations' was a term that Ambassadors Volker and Sondland used to mean matters related to the 2016 elections, and to investigations of Burisma and the Bidens," Taylor also noted, referring to former special envoy Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE. Sondland and Volker have also testified before the House during its impeachment probe.

Read Taylor's full opening statement below: