1st current White House official to cooperate with House impeachment investigators willing to testify publicly: Source Vindman testified before House investigators behind closed doors on Tuesday.

Lt. Col. Alexander Vindman, the first current White House official to cooperate with House impeachment investigators and appear on Capitol Hill for a closed-door deposition, is willing to testify publicly in the next phase of the inquiry, according to a source familiar with his thinking.

A National Security Council official Ukraine specialist, Vindman told investigators earlier this week that he was so "concerned" by the president’s comments on a July phone call with the Ukrainian president, he notified a White House lawyer.

Vindman, the first witness to appear on Capitol Hill with firsthand knowledge of the call, has not yet been formally contacted by House impeachment investigators about testifying in public, the source said.

"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 testified, according to a copy of his opening remarks obtained by ABC News.

An attorney for Vindman and a spokesman for House Intelligence Committee Chairman Adam Schiff declined to comment for this story.

The Ukraine-born Vindman, who serves in the Army and was awarded a Purple Heart after an improvised explosive device attack in Iraq, could be a key witness for Democrats as they seek to take public their investigation into whether President Donald Trump improperly withheld military aid to Ukraine to pressure the country to open investigations into Burisma, an energy company that once employed former Vice President Joe Biden’s son, Hunter, and the 2016 election.

"I realized that if Ukraine pursued an investigation into the Bidens and 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," Vindman said in his opening statement. "This would all undermine U.S. national security. Following the call, I again reported my concerns to NSC’s lead counsel."

The House voted largely along party lines on Thursday to approve an impeachment resolution, laying out the procedures for the upcoming public hearings, which could begin as early as November, in the House Intelligence Committee.

The testimony of Vindman could provide insight into how White House officials work to restrict access to a rough transcript of Trump's call with the Ukrainian president.

Vindman, along with his twin brother Yevgeny, who also serves in the Army and on the NSC staff, approached John Eisenberg, the NSC’s top lawyer, with his concerns about Trump’s comments on the July 25 call and read-out notes he took during it, according to a source familiar with Vindman's testimony.

In that meeting, which also included Michael Ellis, a White House attorney and Eisenberg’s deputy, Eisenberg suggested moving the transcript of Trump’s call with Ukrainian President Volodymyr Zelenskiy to a system meant to store classified, sensitive national security information, according to the source.

House impeachment investigators are seeking to question Ellis and Eisenberg next week, though it's unclear if they will cooperate with the inquiry.

Vindman also testified that he suggested changes to clarify a rough transcript of Trump’s call with Zelenskiy, according to sources present for his testimony.

Two of the changes -- a mention of Burisma, the Ukrainian energy company, by Zelenskiy, and a reference to recordings of Biden discussing corruption in Ukraine by Trump -- were not adopted, Vindman told investigators, though he did not testify to any motive behind the White House’s review process.

Vindman also told impeachment investigators that he believed that the promise of a White House meeting, and later, nearly $400 million in military aid to Ukraine, was being withheld in exchange for the investigations into Burisma and the 2016 election, according to sources familiar with his testimony.

Republicans who participated in the deposition on Wednesday dismissed Vindman’s opening statement and impression of Trump’s call with Zelenskiy.

"It doesn't trouble me," Rep. Jim Jordan, R-Ohio, the top Republican on the House Oversight Committee, told reporters. "The facts have not changed."