Lt. Col. Alexander Vindman, ‘98, appeared voluntarily under congressional subpoena on Tuesday to tell impeachment inquiry investigators on Capitol Hill the details of a July phone call between U.S. President Donald Trump and Ukrainian President Volodymyr Zelensky.

Vindman, top Ukraine expert on the National Security Council (NSC) and the first witness to appear in the impeachment probe, was on the phone call and said Trump undermined U.S. national security when he pressured Zelensky to investigate former Vice President Joe Biden and his son, Hunter Biden.

He also told investigators he raised concerns regarding the call with the NSC’s lead attorney, directly contradicting testimony from Gordon Sondland, U.S. ambassador to the European Union and a Trump appointee. Sondland, who met with impeachment investigators earlier this month, told House investigators that no concerns had been raised about the phone call and defended Trump’s actions.

“I was concerned by the call,” Vindman wrote in his statement to the impeachment committee. “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. 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. This would all undermine U.S. national security.”

Vindman went into the military immediately after graduating from Binghamton University with an undergraduate degree in history, completing basic training at Fort Benning, Georgia in 1999, and deploying to South Korea the following year. Later, he would serve in Iraq, where he was injured by a roadside bomb, and was awarded the Purple Heart. He is a Ukrainian immigrant, and arrived in the United States with his father, grandmother and twin brother at 3 years old. Since 2008, he has been a foreign area officer with the U.S. Army specializing in Eastern European and Asian affairs. He holds a master’s degree from Harvard in Russian, Eastern Europe and Central Asian studies.

Trump took aim at Vindman on Tuesday morning, calling him a “Never Trumper witness” in a tweet. But following attacks from Trump and his allies, several top Republican lawmakers stepped up to defend Vindman on Tuesday afternoon, including Sen. John Thune (R-SD), who called Vindman a “patriot,” and Senate Majority Leader Mitch McConnell (R-KY), who said he would not “question the patriotism of any of the people who come forward.”

Following Vindman’s testimony, House Democrats released a rules package for the impeachment inquiry, and plan to vote on it Thursday. The new rules would make the currently confidential inquiry public, directing the Intelligence Committee to convene open hearings and produce a written report on the findings of its investigation. It also includes measures to implement new due process rights for Trump.

In the meantime, Democratic lawmakers are also subpoenaing more witnesses to testify in the inquiry, including Robert Blair, a top national security adviser to Mick Mulvaney, the acting White House chief of staff. It is unclear whether potential witnesses will comply with the requests.