A former assistant U.S. attorney has said that Lieutenant Colonel Alexander Vindman—set to testify before House committees Tuesday regarding President Donald Trump’s interaction with Ukrainian President Volodymyr Zelenskiy—is a “star witness” whose experience will lend his testimony great significance.

Mimi Rocah, who served as a prosecutor in the Southern District of New York, appeared on MSNBC Monday to give her insights on Vindman’s scheduled appearance. She told host Brian Williams that Vindman “is the kind of witness that prosecutors dream about.”

Vindman was on the now-infamous phone call between Trump and Zelenskiy. The president is accused of using the conversation to propose a quid pro quo of military aid in exchange for a Ukrainian investigation into allegations of corruption against 2020 rival Joe Biden and his son Hunter, linked to Ukraine’s Burisma natural gas company.

A memo detailing parts of the call was released by the White House last month, prompting House Democrats to launch an impeachment investigation into the president. The full transcript of the call has still not been released. “We need to know everything surrounding the call,” Rocah suggested.

Vindman’s status as a first-hand witness is one element of why his testimony could be so valuable, Rocah explained.

“He was on the call,” she told Williams. “So, amongst other things, he can be questioned about what were those ellipses in the summary, is there more? I don’t want to raise expectations because this call is damning as it is, but there are factual questions that he now gets to be asked.”

Because he was on the call, Rocah said Republicans will be unable to argue that he does not have first-hand knowledge of what transpired. She added she thought such a line of attack “was a ridiculous argument anyway, but they can’t make it.”

Vindman is on active duty with the U.S. Army and received a Purple Heart for wounds sustained during a combat tour in Iraq. He is also the director for European affairs on the National Security Council and was part of the U.S. delegation that traveled to Kiev for Zelenskiy’s inauguration in May.

This delegation also included Energy Secretary Rick Perry, Ukraine special envoy Kurt Volker and Ambassador to the European Union Gordon Sondland, who are all central to the House probe into the White House’s Ukraine strategy.

Such experience lends Vindman’s testimony even more weight, Rocah said. “He’s active-duty military,” she told William, suggesting this would play very well with Fox News. “We also have a person…of impeccable character and that’s a pretty good witness to have.”

Williams asked Rocah, “Counselor, is this what you would call in the law a star witness?” She quickly replied, “Absolutely.”

Vindman’s opening statement, leaked to multiple organizations Monday, shows he will tell investigators that he was deeply troubled by Trump’s call with Zelenskiy and reported his concerns to a superior.

“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’s written statement explains.

“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,” the statement continues.

“This would all undermine U.S. national security. Following the call, I again reported my concerns to NSC’s lead counsel.”