The top U.S. envoy to Ukraine undermined President Donald Trump‘s denial of any quid pro quo agreement between the U.S. and Ukraine, testifying Tuesday to House impeachment investigators that Trump withheld military aid and refused a White House meeting with the Ukrainian president unless he agreed to launch a political investigation.

William Taylor, a former ambassador to Ukraine himself, was appointed to the position in June after former ambassador Marie Yovanovitch was recalled.

During his prepared opening remarks in the closed-door testimony, Taylor described finding “a weird combination of encouraging, confusing, and ultimately alarming circumstances” when he first arrived in Ukraine.

He implicated Special Envoy to Ukraine Kurt Volker, U.S. ambassador to the European Union Gordon Sondland, Secretary of Energy Rick Perry, and Trump attorney Rudy Giuliani as operating in Ukraine outside of the State Department. He testified that he was included in some conversations, but otherwise was not a part of the group.

“On June 27, Ambassador Sondland told me during a phone conversation that President Zelenskyy needed to make clear to President Trump that he, President Zelenskyy, would not stand in the way of ‘investigators,'” said Taylor, describing an exchange in which Trump refused a White House meeting with Ukraine.

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Taylor continued, outlining a quid pro quo deal Trump pushed involving granting the Ukrainian president a White House meeting in exchange for opening a political probe.

“By mid-July it was becoming clear to me that the meeting President Zelenskyy wanted was conditioned on the investigations of Burisma and alleged Ukrainian interference in the 2016 U.S. elections,” Taylor said.

Hunter Biden, the son of Trump’s political opponent Joe Biden, sat on the board of the gas company Burisma. His demand for a probe was seen by some, as a direct request to a foreign government to investigate a political rival.

Trump, who urged Ukraine to investigate Biden in a July 25 phone call, has consistently denied the allegation that military aid was used as leverage.

However, in the nearly 10-hour long testimony, Taylor continued to emphasize that Trump would not meet with Ukraine until they agreed to his request. He also told impeachment investigators that a White House budget official said during a July phone that Trump had personally directed that the military aid be withheld.

Rep. Stephen Lynch (D-Massachusetts) called the testimony as a “sea change” that “could accelerate” the impeachment inquiry.

White House Press Secretary Stephanie Grisham described the testimony as “just more triple hearsay and selective leaks from the Democrats’ politically-motivated, closed door, secretive hearings.”

Stephen Sestanovich, a former ambassador at large to the former Soviet Union who has traveled in Ukraine with Taylor, told the New York Times that Taylor is a highly credible source.

“You couldn’t ask for a more credible, universally respected, upright public servant to testify on the facts of this case,” he said. “You want to go against Bill Taylor, you’ve got the whole city against you.”