WASHINGTON — Defying White House orders, an Army officer serving with President Donald Trump’s National Security Council testified to impeachment investigators Tuesday that he twice raised concerns over Trump’s push to have Ukraine investigate Democrats and Joe Biden.

Alexander Vindman, a lieutenant colonel who served in Iraq and later as a diplomat, told House investigators behind closed doors that he listened to Trump’s July 25 call with new Ukrainian President Volodymyr Zelenskiy and reported his concerns to the NSC’s lead counsel, according to his prepared testimony.

His arrival in military blue, with medals, created a striking image as he entered the Capitol and made his way to the secure briefing room.

“I was concerned by the call,” Vindman said, according to his testimony obtained by The Associated Press. “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, a 20-year military officer, was the first official who listened in on the phone call to testify as the impeachment inquiry reaches deeper into the Trump administration and Democrats prepare for the next, public phase of the probe. He was also the first current White House official to appear before the impeachment panels. With the administration directing staff not to appear, he was issued a subpoena to testify.

The inquiry is looking into Trump’s call, in which he asked Zelenskiy for a “favor” — to investigate Democrats — that the Democrats say was a quid pro quo for military aid and could be an impeachable offense.

Trump took to Twitter Tuesday to denounce the probe as a “sham,” adding: “Why are people that I never even heard of testifying about the call. Just READ THE CALL TRANSCRIPT AND THE IMPEACHMENT HOAX IS OVER!”

Vindman, who arrived in the United States as a 3-year-old from the former Soviet Union, wrote that it was his “sacred duty” to defend the United States.

Some Trump allies, looking for ways to discredit Vindman, questioned the colonel’s loyalties because he was born in the region. But the line of attack was rejected by some Republicans, including Rep. Liz Cheney, who said it was “shameful” to criticize his patriotism.

His appearance came a day after Speaker Nancy Pelosi announced the House will vote on a resolution to affirm the impeachment investigation, set rules for public hearings and outline the potential process for writing articles of impeachment against Trump. The vote is expected Thursday.

Vindman said that in spring of this year he became aware of “outside influencers” promoting what he called a “false narrative of Ukraine” that undermined U.S. efforts. Ukraine, in trying to become a vibrant democracy integrated with the West, is a bulwark against overt Russian aggression, he said in his opening statement.

Other officials have testified that Ukraine policy was increasingly being handled by the president’s personal attorney, Rudy Giuliani, and others in the administration outside regular policy-making channels. One diplomat testified it was “highly irregular.”