Democrats on Tuesday announced a second week of open hearings in the House impeachment inquiry, including with E.U. Ambassador Gordon Sondland, former Ukraine special envoy Kurt Volker, and top Ukraine expert Lt. Col. Alexander Vindman.

Sondland, Volker and Vindman are key figures in the House impeachment inquiry into President Donald Trump. Sondland has acknowledged delivering a quid pro quo message to Ukraine, while Volker had numerous dealings with Trump's personal lawyer, Rudy Giuliani.

Testimony from Vindman, the Director for European Affairs at the National Security Council, is also crucial as he was present during the July phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate the Bidens. Vindman told impeachment investigators in his closed-door deposition last month that there was "no doubt" what Trump was intending to do.

"It doesn't take a rocket scientist to see where the gain would be for the president in investigating the son of a political opponent," Vindman said, according to the transcript of his closed-door testimony made public last week.

In a press release, the committee announced they would also hear testimony from Jennifer Williams, an aide to Vice President Mike Pence, on the morning of Tuesday, Nov. 19 alongside Vindman. That same afternoon, the committee will hear testimony from Volker and Tim Morrison, a White House aide with the National Security Council.

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The next morning, the committee will hear testimony from Sondland. The committee will hear from Laura Cooper, the Deputy Assistant Secretary of Defense for Russian, Ukrainian, and Eurasian Affairs and David Hale, the Under Secretary of State for Political Affairs, later in the day on Wednesday.

Wednesday's hearings will take place hours before the fifth Democratic presidential debate, hosted by MSNBC and The Washington Post.

Thursday morning, the panel will hear testimony from Fiona Hill, the former National Security Council senior director for Europe and Russia who testified that Sondland had told Ukrainian officials they needed to proceed with "investigations" in order to line up a White House visit for Ukraine's president.

Volker and Morrison were among the witnesses who the Republicans on the committee had said they wanted to testify. Morrison said in a closed door deposition on Halloween he thought there was "nothing illegal" about Trump's phone conversation with Zelenskiy, sources have told NBC News.