ABC News November 19, 2019

Read opening statements of Lt. Col. Vindman and others testifying on Day 3 of public impeachment hearings

WATCH: Vindman says he believes Trump ‘ordered’ Zelenskiy to investigate Bidens

Kurt Volker, former special envoy to Ukraine, and Tim Morrison, a departing member of the National Security Council, will testify publicly before the House Intelligence Committee Tuesday afternoon, following morning testimonies by Lt. Col. Alexander Vindman, the National Security Council's Ukraine expert, and Jennifer Williams, a national security aide to Vice President Mike Pence.

Volker was the first witness to testify voluntarily -- without a subpoena -- behind closed doors in the impeachment proceedings.

Morrison, who listened in on President Donald Trump’s July 25 phone call with Ukrainian President Volodymyr Zelenskiy, said in closed-door testimony that he heard nothing illegal on the call, although he was concerned that, if it leaked, there could be political fallout.

Vindman and Williams, who also listened in on the call which sparked the impeachment probe, were the first current White House officials to testify publicly in the Democrats' impeachment investigation.

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National Security Council aide Lt. Col. Alexander Vindman testifies before the House Intelligence Committee on Capitol Hill in Washington, D.C., Nov. 19, 2019.

Read Vindman's opening statement:

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Jennifer Williams, an aide to Vice President Mike Pence, testifies on Capitol Hill in Washington, D.C. on Nov. 19, 2019.

Read Williams' opening statement:

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Former State Department special envoy to Ukraine Kurt Volker waits to testify before the House Intelligence Committee on Capitol Hill, Nov. 19, 2019.

Read Volker's opening statement:

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Tim Morrison, a former official at the National Security Council arrives to testify before the House Intelligence Committee on Capitol Hill, Nov. 19, 2019, during a public impeachment hearing of President Donald Trump's efforts to tie U.S. aid for Ukraine to investigations of his political opponents.

Read Morrison's opening statement: