WASHINGTON - In a significant revision to his testimony nearly three weeks ago before House impeachment investigators, the U.S. ambassador to the European Union, Gordon Sondland, now says he told a Ukrainian official that security assistance to the country would be likely to resume only if the authorities in Kyiv opened investigations requested by President Donald Trump that could be damaging to former vice president Joe Biden.

In a "supplemental declaration" provided to the House impeachment inquiry Monday, Sondland wrote, "I now recall speaking individually" with a Ukrainian official and in that conversation saying "that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks."

Sondland's new statement adds to testimony by other national security officials that describes an effort directed by Trump and his personal attorney Rudy Giuliani to link nearly $400 million in security assistance to investigations that could politically benefit the president.

Sondland, a Trump donor turned diplomat, had been seen as a loyalist of the president with a supportive version of events - that Trump was trying to combat corruption broadly in a country that depends on U.S. aid to defend itself against Russian aggression. Sondland's assertion in a previously released text message to a senior State Department official that Trump didn't seek "quid pro quo's of any kind" was seized upon by Republicans to argue that the president had not used the power of his office for personal political gain.

In his testimony last month, Sondland undercut part of that argument, telling congressional investigators that he texted the phrase only after it was relayed to him directly by Trump. He also said he had been working to carry out one quid pro quo - bartering a White House visit coveted by Ukraine's new president, Volodymyr Zelensky, for assurances his administration would carry out the investigations Giuliani demanded, including an inquiry into an energy company, Burisma, where Biden's son Hunter held a board seat.

His revised statement adds to the testimony of other senior national security officials who told lawmakers that security assistance was also used to try to compel the Ukrainians to open investigations that might help Trump in 2020.

A White House spokeswoman dismissed Sondland's statements, as well as those made on Oct. 3 by former U.S. envoy to Ukraine, Kurt Volker, whose deposition was released Tuesday, along with Sondland's.

The transcripts of their testimony "show there is even less evidence for this illegitimate impeachment sham than previously thought," White House press secretary Stephanie Grisham said in a statement. "Ambassador Sondland squarely states that he 'did not know, (and still does not know) when, why or by whom the aid was suspended.' He also said he 'presumed' there was a link to the aid - but cannot identify any solid source for that assumption."

Also Tuesday, the impeachment committees announced they had requested acting White House chief of staff Mick Mulvaney appear on Saturday. The agency he also oversees, the Office of Management and Budget, had a significant role in the withholding of aid to Ukraine.

In his opening statement to the House last month, Sondland said he had no knowledge of whether the White House was also holding up security assistance to press for the investigations. He was asked during his testimony, "So, you've never made a statement relating the aid to conditions that the Ukraine ought to comply with?"

"I don't remember that, no," Sondland responded.

Sondland's testimony was later undercut in subsequent depositions by U.S. officials.

The following week, William Taylor Jr., the acting U.S. ambassador to Ukraine, challenged Sondland's claim. Taylor testified that Sondland, in a meeting with Ukrainian officials in Poland in September, had conditioned the release of the funding on an investigation targeting the Bidens.

Taylor said he understood that on Sept. 1, Sondland warned Zelensky aide Andrey Yermak that the security assistance "would not come" unless the new Ukrainian president committed to pursuing the investigation of Burisma.

"I was alarmed," Taylor wrote, saying a national security official had told him the demand was relayed in person by Sondland while the ambassador was traveling in Poland with Vice President Mike Pence. "This was the first time I had heard that the security assistance . . . was conditioned on the investigation."

After the first revelations of such an exchange in Taylor's testimony, Sondland attorney Robert Luskin wrote to The Washington Post on Oct. 23, saying that his client "does not recall" such a conversation.

Sondland now says the testimony of Taylor and others "refreshed my recollection about conversations involving the suspension of U.S. aid."

In his new declaration, Sondland stated that "by the beginning of September 2019, and in the absence of any credible explanation for the suspension of aid, I presumed that the aid suspension had become linked" to Ukraine having not yet committed publicly to the investigation of Burisma and another into a discredited theory about Ukrainian involvement in the 2016 U.S. election.

"It would have been natural for me to have voiced what I had presumed," Sondland said, acknowledging that he told Yermak that "resumption" of U.S. aid would probably not occur until Ukraine provided the public anti-corruption statement that the officials had been discussing.

Sondland and Volker testified about Trump's long-held suspicions of Ukraine and the unorthodox role Giuliani played in crafting a U.S. policy.

In a meeting with Trump in the Oval Office on May 23, Volker, Sondland and Energy Secretary Rick Perry tried to persuade Trump to engage with Ukraine's new president, Zelensky, whom they had just met after his inauguration.

Trump would hear none of it, Sondland testified.

"The president was railing about Ukraine . . . he was going on and on and on about his dissatisfaction with Ukraine," Sondland testified. "He didn't even want to deal with it anymore. And he basically waved and said: 'Go talk to Rudy, he knows all about Ukraine.' "

Sondland, Volker and Perry were disappointed about having to work with Giuliani because it was abnormal and was another impediment to scheduling a Trump meeting with Zelensky, Sondland said.

"Until Rudy was satisfied, the president wasn't going to change his mind," he said.

Sondland negotiated with Ukrainian officials in August to craft a press statement that said they would investigate corruption, as part of a bid to schedule a meeting between the two leaders, he said.

The initial draft, he said, had no specific commitments: "It just said corruption per se."

Later, he said, Giuliani added new conditions about referencing Burisma, the Ukrainian company that employed Biden's son.

"Mr. Giuliani was the one giving the input as to what the president wanted in the statement," Sondland testified. "He wanted Burisma and 2016 election mentioned in the statement.

Volker testified: "Rudy says, 'Well, if it doesn't say Burisma and if it doesn't say 2016, what does it mean? You know, it's not credible. You know, they're hiding something.' "

"I would not endorse investigating the Bidens," Sondland said, adding the Ukrainians were not "prepared" to do that.

The statement was never issued.

As the pressure campaign on Ukraine continued, Trump became more frustrated. In one exchange, Sondland described the president as "in a very bad mood" when Sondland called to ask what Trump hoped to achieve by pressing Ukraine on negotiations.

The call took place moments after Taylor had raised sharp concerns in a text message with Sondland about a possible illicit quid pro quo regarding aid to Ukraine.

Taylor texted Sondland and Volker on Sept. 9: "I think it's crazy to withhold security assistance for help with a political campaign."

That prompted Sondland's call to Trump, which he described to investigators:

"There were all kinds of rumors. And I know in my few previous conversations with the President, he's not big on small talk, so I would have one shot to ask him. And rather than asking him, 'Are you doing X because of X or because of Y or because of Z?' I asked him one open-ended question: What do you want from Ukraine?"

The president replied, "I want nothing. I want no quid pro quo. I want Zelensky to do the right thing," according to Sondland's account.

"And I said: 'What does that mean?' And he said: 'I want him to do what he ran on.' And that was the end of the conversation," Sondland said. "I wouldn't say he hung up me, but it was almost like he hung up on me."

About five hours after Taylor's text, Sondland wrote back to Taylor: "The President has been crystal clear no quid pro quo's of any kind."

"I had gotten as far as I could," Sondland told House investigators. "I had asked the boss what he wanted. He wouldn't tell me, other than: I want nothing."

Republican counsel Steve Castor asked Sondland whether Trump told him to write the "no quid pro quo's" text.

"The President didn't know I was sending a text," Sondland replied, "because he didn't know that the question came from Ambassador Taylor."

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The Washington Post's Devlin Barrett, Mike DeBonis, Josh Dawsey, Karoun Demirjian, Rosalind S. Helderman, John Hudson, Greg Jaffe and Elise Viebeck contributed to this report.

This article was written by Shane Harris and Aaron C. Davis, reporters for The Washington Post.