On Thursday evening, the House of Representatives Intelligence Committee released text messages between Kurt Volker, the former U.S. special envoy to Ukraine, and Ukrainian government officials. The conversations focused on the withholding of military aid to Ukraine, arranging a meeting between Trump and Ukranian President Volodymyr Zelensky and the 2016 election.

On Friday morning, Katyal wrote on Twitter that he was waking up to “all of this new evidence.” He claimed it showed impeachment was “constitutionally required” and called it “damning and sad.”

“This is just the first week of the investigation,” Katyal wrote, “Trump’s factual defense has already collapsed.” Katyal, who was once named “Litigator of the Year” by American Lawyer magazine, has argued more cases before the U.S. Supreme Court than any other minority attorney, according to his firm’s website. He recently surpassed the previous record set by Thurgood Marshall.

Katyal reiterated his opinion during an interview with NPR released on Friday. Katyal, who served in former President Barack Obama’s administration, told Morning Edition host Steve Inskeep that the text messages corroborated the Ukraine whistleblower complaint. He added that it adds “color” to Trump’s phone call with the Ukranian president.

“I think this is the quintessential definition to our founders of what is impeachable,” Katyal said.

In a July text message, Volker told Ukrainian adviser Andrey Yermak that he heard from the White House and as long as Zelensky convinced Trump he’d investigate what happened in 2016, a date for a visit to Washington, D.C., would be “nailed down.”

During a phone call with Zelensky, Trump noted that there was a lot of talk about former Vice President Joe Biden’s son, Hunter Biden, who served on the board of the gas company Burisma, which was previously the topic of a Ukrainian investigation.

“That Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great,” Trump said according to a transcript of the call. “Biden went around bragging that he stopped the prosecution so if you can look into it. … It sounds horrible to me.” The call took place as Joe Biden was emerging as the frontrunner for the Democratic nomination in the 2020 presidential race, in which he will likely run against Trump.

Trump recently reiterated his desire for Ukraine, as well as China, to investigate the former vice president and his son, despite the fact that his initial comments about the Biden family during the call with Zelensky launched the impeachment inquiry currently underway. The president has criticized the inquiry as a waste of time.

On Friday morning, Trump posted on Twitter that as president, he had an “obligation to end corruption.” That obligation, Trump wrote, could mean requesting the help of foreign countries.

“It is done all the time. This has NOTHING to do with politics or a political campaign against the Bidens. This does have to do with their corruption!” Trump wrote on Twitter.

While the president’s been criticized for inappropriately pressuring a foreign government to meddle in a U.S. election, Ukrainian Foreign Minister Vadym Prystaiko denied that was the case.

“I know what the conversation was about and I think there was no pressure,” Prystaiko said, according to Reuters. “This conversation was long, friendly, and it touched on many questions, sometimes requiring serious answers.”