National Security Council Ukraine expert Lt. Col. Alexander Vindman (C) arrives for a closed-door meeting with House committees conducting an impeachment inquiry against President Donald Trump. Photo by Pete Marovich/UPI | License Photo

Oct. 29 (UPI) -- A top White House official testified Tuesday that he does not know the identity of the whistle-blower who reported that President Donald Trump pressured the Ukrainian president to dig up dirt on Trump's political rival, former Vice President Joe Biden and his son.

Lt. Col. Alexander Vindman, a Ukraine policy expert for the National Security Council, appeared before the House intelligence, foreign affairs, and oversight and reform committees to answer questions about whether Trump threatened to withhold military aid to Ukraine in exchange for investigations involving Biden's son, Hunter Biden, who worked for a Ukrainian gas company.


Vindman monitored the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky, which resulted in a whistle-blower complaint that has sparked a Trump impeachment inquiry.

During Tuesday's testimony, Hous Intelligence committee Chairman Adam Schiff, D-Calif., objected to a line of questioning from Republicans, stating they were part of an effort to reveal the whistle-blower's identity, CNN reported.

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Vindman testified he did not know who the whistle-blower was.

"What Republicans are trying to do in there, very clearly in their questioning, is try to front-door or backdoor Lt. Col. Vindman into revealing who the whistleblower is, ecen though in his testimony, he said he didn't know," Rep. Debbie Wasserman Schultz, D-Fla., said. "They are trying to, through the back door and through process of elimination by their questions, they are attempting to get him to reveal that and they have been unsuccessful."

Rep. Mark Meadows, R-N.C., said the accusation that Republicans were attempting to reveal the whistle-blower's identity was inaccurate.

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"I've not been on any fishing expedition," he said. "I only ask questions we know the answer to."

In prepared remarks obtained by Politico and ABC News, Vindman stated that Trump undermined national security in the July phone 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 wrote.

Vindman was the first White House official to testify in the impeachment proceedings. The Trump administration has made efforts to block other officials from giving depositions.

"I realized that if Ukraine pursued an investigation into the Bidens and [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," Vindman's statement read.

"This would all undermine U.S. national security," he added. "Following the call, I again reported my concerns to NSC's lead counsel."

Trump reacted to reports about Vindman's testimony, referring to him as a "Never Trumper" and questioning his account of the call.

"Was he on the same call that I was? Can't be possible! Please ask him to read the transcript of the call," Trump wrote on Twitter.

Vindman's testimony follows that of other key witnesses, including U.S. Ambassador to Ukraine William Taylor and U.S. Ambassador to the European Union Gordon Sondland. In his remarks, Vindman said Sondland had stressed to him the importance of Ukraine's cooperation, and that he'd been supported by former national security officer Fiona Hill.

"I stated to Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push," he wrote.

"Following the debriefing meeting, I reported my concerns to the NSC's lead counsel. Dr. Hill also reported the incident to the NSC's lead counsel."

Vindman said in his statement that he worried the dealings would undermine U.S. interests in eastern Europe.

He said he decided to testify out of a "sense of duty."

"I am a patriot, and it is my sacred duty and honor to advance and defend our country, irrespective of party or politics," Vindman wrote. "For over 20 years, as an active duty United States military officer and diplomat, I have served this country in a nonpartisan manner and have done so with the utmost respect and professionalism for both Republican and Democratic administrations."