A high-ranking White House national security official will tell congressional impeachment investigators on Tuesday that he was so alarmed about a call in which U.S. President Donald Trump pushed Ukraine’s leader to investigate former vice president Joe Biden that he reported it to a top government lawyer.

It was the second time in less than a month that Alexander Vindman, an U.S. Army lieutenant colonel who sits on the National Security Council (NSC) and who listened in on the call, had contacted the lead NSC counsel about what he saw as inappropriate requests of Ukraine, according to his prepared testimony, seen by Reuters on Monday.

In the prepared remarks, Vindman said that if President Volodymyr Zelenskiy did as Trump asked in the July 25 call and investigated Biden, it would “undermine U.S. national security.”

Vindman, a veteran who was awarded a Purple Heart, will be the first current White House official to testify in the House of Representatives’ impeachment inquiry, which was largely prompted by a whistleblower report on the call between Trump and Zelenskiy.

On the July 25 call, Trump pressed Zelenskiy to investigate Biden, a former vice president and a leading candidate for the Democratic presidential nomination to run against Trump in 2020.

“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 prepared statement.

He voiced similar concerns to the NSC’s lead counsel after a meeting on July 10, his prepared remarks say, as did Fiona Hill, a former NSC adviser on Russia who has testified to impeachment investigators.

At that meeting, Vindman said the U.S. ambassador to the European Union, Gordon Sondland, told Ukrainian officials that they needed to “deliver specific investigations in order to secure a meeting with the President,” at which point National Security Advisor John Bolton cut the meeting short.

After the meeting, according to Vindman’s prepared remarks, Sondland said it was important that the Ukrainian investigations center on the 2016 election, the Bidens and Burisma, a Ukrainian energy company whose board included Biden’s son Hunter.

The president’s personal lawyer, Rudy Giuliani, and others have made discredited allegations that when Biden was vice president, he had a prosecutor fired to halt an investigation into Burisma.

Both Vindman and Hill told Sondland his statements in the meeting were “inappropriate,” according to Vindman’s prepared remarks.

Vindman’s testimony is at odds with that of Sondland, who spoke to congressional investigators in closed hearings this month. Sondland told them he did not understand “until much later” that Burisma was connected to the Bidens.

Sondland said in his opening statement to them that he did not recall taking part in any effort to encourage an investigation into the Bidens.

Vindman could be an important witness in the probe. His statement notes his two decades in the Army, including combat in Iraq, where he was wounded.

“I am a patriot and it is my sacred duty and honor to advance and defend OUR country, irrespective or party or politics,” Vindman says.