READ: William Taylor's opening statement at impeachment hearing

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 TrumpDonald TrumpJan. 6 panel faces double-edged sword with Alex Jones, Roger Stone Trump goes after Woodward, Costa over China Republicans seem set to win the midterms — unless they defeat themselves 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 BidenJoe BidenSouth Africa health minister calls travel bans over new COVID variant 'unjustified' Biden attends tree lighting ceremony after day out in Nantucket Senior US diplomat visiting Southeast Asia to 'reaffirm' relations 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 VolkerKurt VolkerThe Hill's Morning Report - Presented by National Industries for the Blind - Schumer: Dem unity will happen eventually; Newsom prevails CNN obtains audio of 2019 Giuliani call linked to Ukraine meddling allegations GOP senators request details on Hunter Biden's travel for probe MORE. Sondland and Volker have also testified before the House during its impeachment probe.  

Read Taylor's full opening statement below: