The acting U.S. ambassador to Ukraine, William Taylor, testified on Tuesday that he was told the delivery of military aid to Ukraine was contingent on Ukraine promising to investigate corruption allegations against former vice president Joe Biden and his son Hunter, according to the Washington Post.

Taylor had previously sent a message to Gordon Sondland, ambassador to the EU, on September 9, writing: “As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign.”

“During that phone call, Ambassador Sondland told me that President Trump had told him that he wants [Ukrainian President Volodymyr] Zelensky to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 election,” Taylor told House impeachment investigators. Hunter Biden was a former board member of Burisma, a Ukrainian natural-gas company, while his father was vice president.

“Ambassador Sondland also told me that he now recognized that he had made a mistake by earlier telling the Ukrainian officials to whom he spoke that a White House meeting with President Zelensky was dependent on a public announcement of investigations — in fact, Ambassador Sondland said, ‘everything’ was dependent on such an announcement, including security assistance,’” Taylor continued.

“He said that President Trump wanted President Zelensky ‘in a public box’ by making a public statement about ordering such investigations.”

“It was just the most damning testimony I’ve heard,” commented Rep. Debbie Wasserman Schultz (D., Fla.).

Acting White House chief of staff Mick Mulvaney had told reporters on Thursday that Trump had ordered a halt to military aid to Ukraine to pressure the country to investigate interference in the 2016 U.S. presidential elections. Trump and allies have repeatedly alleged that evidence for that interference can be found on a Ukrainian server for Crowdstrike, a cyber security company, following a theory that has been repeatedly debunked.

When a reporter said Mulvaney was describing a “quid pro quo,” Mulvaney responded, “We do that all the time with foreign policy.”

The next day Mulvaney walked back his comments.

“Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election,” Mulvaney said. “The president never told me to withhold any money until the Ukrainians did anything related to the server.”