A White House National Security Council official who listened to President Donald Trump's phone call with Ukraine's leader plans to tell House impeachment investigators in a sworn deposition Tuesday that he was so alarmed by the conversation that he reported it to his superiors and the NSC's top lawyer.

Lt. Col. Alexander Vindman, an active-duty Army officer and the NSC's top Ukraine expert, will be the first White House official to testify as part of House Democrats' impeachment inquiry into Trump. Vindman will also be the first official who heard Trump's call with Ukrainian President Volodymyr Zelensky to testify before the committees leading the impeachment probe.

"The facts are damning. The facts must be followed, and the law. And the committees must do their work and the Congress must do its constitutional duty."

—Rep. Sheila Jackson Lee

"I was concerned about the call," Vindman plans to say, according to a copy of his opening statement (pdf) obtained by Politico.

"I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine," Vindman will say, referring to Trump's request that Ukraine launch an investigation into former Vice President Joe Biden and his son Hunter. "This would all undermine U.S. national security."

Rep. Sheila Jackson Lee (D-Texas.), a member of the House Judiciary Committee, said in a statement that Vindman's "damning testimony... reinforces the fact that the president of the United States sought to have Ukraine provide dirt" on Biden.

"His efforts were not for the people of the United States, not for the safety and security of Ukraine, but for simply and purely selfish reasons to win a presidential election," said Jackson Lee. "The facts are damning. The facts must be followed, and the law. And the committees must do their work and the Congress must do its constitutional duty."

In addition to corroborating the details of the whistleblower complaint prompted by Trump's call with Zelensky, Vindman's opening remarks also appear to contradict the sworn testimony of Gordon Sondland, U.S. Ambassador to the European Union.

Vindman plans to say he told Sondland that Trump's effort to pressure Ukraine to launch an investigation into Biden was "inappropriate."

Earlier this month, Sondland told House impeachment investigators that no one on NSC staff "ever expressed any concerns to me about our efforts."

Sondland testimony directly contradicted by Vindman. According to Vindman, after Sondland told the Ukrainians that it was important they launch Biden/Burisma investigations, Fiona Hill “entered the room and asserted to Amb. Sondland that his statements were inappropriate.” https://t.co/xw0IQJk1nw — Natasha Bertrand (@NatashaBertrand) October 29, 2019

Rep. Joaquin Castro (D-Texas), a member of the House Foreign Affairs Committee, tweeted that the evidence suggests Sondland is guilty of a crime.

"Based on all the testimony so far," said Castro, "I believe that Ambassador Gordon Sondland committed perjury."