Washington • A crucial witness in the impeachment inquiry reversed himself this week and acknowledged to investigators that he had told a top Ukrainian official that the country would most likely have to give President Donald Trump what he wanted — a public pledge for investigations — in order to unlock military aid.

The disclosure from Gordon D. Sondland, an ally of Trump who is the United States ambassador to the European Union, confirmed his role in laying out a quid pro quo to Ukraine that conditioned the release of security assistance from the United States on the country’s willingness to say it was investigating former Vice President Joe Biden and other Democrats.

That admission, included in a four-page sworn statement released Tuesday, directly contradicted his testimony to investigators last month, when he said he “never” thought there was any precondition on the aid.

“I said that resumption of the U.S. aid would likely not occur until Ukraine provided the public anticorruption statement that we had been discussing for many weeks,” Sondland said in the new statement, which was made public by the House committees leading the inquiry, along with the transcript of his original testimony.

Sondland’s disclosure appeared intended to insulate him from accusations that he intentionally misled Congress during his earlier testimony, in which he frequently said he could not recall key details and events under scrutiny by impeachment investigators.

The question of a quid pro quo is at the heart of the impeachment investigation into Trump, which turns on whether the president abused his power when he asked a foreign power to target his political rivals.

The new information surfaced as the House committees also released a transcript of their interview last month with Kurt Volker, the former special envoy to Ukraine.

Rushing to complete their final round of requests for key witnesses before they commence public impeachment hearings, the panels also scheduled testimony Friday by Mick Mulvaney, the acting White House chief of staff, who quickly said he would not comply. And two more administration witnesses who had been scheduled to testify Tuesday — Michael Duffey, a top official in the White House budget office, and Wells Griffith, a senior aide to Energy Secretary Rick Perry — failed to appear.