Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, the first current White House official to be deposed in the House’s impeachment inquiry against 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, is reportedly willing to testify publicly as the House shifts to public hearings.

Vindman, a Ukraine specialist on the National Security Council (NSC), has not yet been formally contacted by House impeachment investigators about public testimony but has said he is willing to, ABC News reported Thursday, citing a source familiar with his thinking.

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In his testimony earlier this week, Vindman testified that upon hearing a call between Trump and Ukrainian President Volodymyr Zelensky in which Trump urged 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 Hunter Biden, he was concerned enough that he contacted a White House lawyer.

"I was concerned by the call. 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 said in his opening statement.

"I realized that if Ukraine pursued an investigation into the Bidens and [natural gas company] Burisma, it would likely be interpreted as a partisan play, which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained," he added.

Vindman is the first witness to have been personally present for the call. He reportedly testified that he took his concerns to John Eisenberg, the NSC’s top lawyer, during a meeting that also included NSC attorney Michael Ellis. Eisenberg reportedly suggested moving the call transcript to a system for classified, sensitive national security information.

The House is seeking to depose Ellis and Eisenberg next week, although it is unclear whether they will comply, according to ABC.

The Hill has reached out to Vindman's attorney for comment.