U.S. lawmakers will question David Holmes, an official from the American embassy in Ukraine, on Thursday as they seek to learn more about a phone call in which he says he overheard President Donald Trump ask about the status of an "investigation" into a political rival.

The Democratic-led House Intelligence Committee will also hear from Fiona Hill, the former senior director for European and Russian Affairs on Trump's National Security Council.

The public impeachment hearing marks the last scheduled day of marathon sessions by the Democratic-led House Intelligence Committee focused on whether Trump wrongfully pressured Ukraine's President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, a Democrat bidding to face Trump in the 2020 election.

What will Hill say?

Hill will warn lawmakers in the House of Representatives impeachment inquiry against promoting falsehoods that minimize Russia's attempts to interfere in U.S. elections. 

According to her prepared testimony, Hill said she has heard questions and statements from some members of the committee that show they appear to believe Russia did not conduct a campaign against the United States during the 2016 presidential race. 

"This is a fictional narrative that has been perpetrated and propagated by the Russian security services themselves," Hill will say. 

Some Republican members of the committee have advanced a discredited conspiracy theory, embraced by Trump, that Ukraine, not Russia, interfered in the last presidential election. 

"In the course of this investigation, I would ask that you please not promote politically driven falsehoods that so clearly advance Russian interests," she will say. 

What does Holmes claim? 

Holmes previously told lawmakers in closed-door testimony that he heard Trump's voice on a July 26 phone call with Gordon Sondland, the U.S. ambassador to the European Union, in which the Republican president asked about Ukraine's willingness to carry out an unspecified investigation. 

"So, he's gonna do the investigation?" Trump asked Sondland, referring to Zelenskiy, according to Holmes' previous testimony. 

"He's gonna do it," replied Sondland, according to Holmes. 

Sondland added the Ukrainian president would do "anything you ask him to," Holmes said. 

Holmes' account ties Trump directly to an effort to get Ukraine to launch an investigation, though his recounting of the overheard telephone call does not explicitly cite the Bidens.

In his November 15 closed door testimony, Holmes said that after overhearing Sondland's phone conversation with Trump at an outdoor restaurant in Kiev, he asked the ambassador if it was true that the president did not care about Ukraine.

In Holmes' telling, Sondland said that it was, and added that Trump only cares about "'big stuff' that benefits the President, like the 'Biden investigation' that Mr. Giuliani was pushing.”

Testifying before the impeachment inquiry on Wednesday, Sondland said he "followed the president's orders" to work with Trump's personal lawyer, Rudy Giuliani, who in turn was pushing Ukraine to carry out two probes that could aid Trump's 2020 re-election campaign.

Sondland told House impeachment investigators he worked with Giuliani on Ukraine at the "express direction" of Trump and pushed a "quid pro quo" with Kyiv because it was what Trump wanted.

The most highly anticipated witness in the public impeachment probe made clear that he believed Trump was pursuing his desire for investigations in return for the Oval Office meeting that the Eastern European nation's president sought.

He said he later came to believe military aid for Ukraine was also being held up until the investigations were launched.

Sondland described how demands became more serious with more conditions on any potential Ukraine meeting at the White House.

"As time went on, more specific items got added to the menu – especially Burisma and 2016 meddling," he said, referring to the gas company where Joe Biden's son Hunter served on the board. And, he added, "the server," the hacked Democratic computer system. 

"I know that members of this committee have frequently framed these complicated issues in the form of a simple question: Was there a 'quid pro quo?' As I testified previously, with regard to the requested White House call and White House meeting, the answer is 'yes'," he said. 

Sondland said he didn't know at the time that Burisma was linked to the Bidens, but has since come to understand that. 

"We had been hearing about it from Rudy and presumed Rudy was getting it from the president."

How has Giuliani reacted?

Giuliani said on Wednesday he had no involvement with the issue of military aid to Ukraine, a key point in the impeachment inquiry into Trump, according to an interview.

Asked on Blaze TV if he ever discussed the U.S. aid, withheld by the Trump administration over the summer, with American diplomats, Trump's personal lawyer said: "The reality is the whole issue of military aid didn't come up until I finished the assignment they gave me. I never discussed military aid with them."

Sondland, a wealthy hotelier and Trump donor, has emerged as a central figure in an intense week in the impeachment probe that has featured nine witnesses testifying over three days. Both Democrats and Republicans were uncertain about what Sondland would testify to, given that he had already clarified parts of his initial private deposition before lawmakers. 

His opening statement included several key details. He confirmed that he spoke with Trump on a cellphone from a busy Kyiv restaurant the day after the president prodded Ukraine's leader to investigate Biden. He also said he kept Secretary of State Mike Pompeo and other top administration officials aware of his dealings with Ukraine on the investigations Trump sought. 

Sondland said he specifically told Vice President Mike Pence he "had concerns" that U.S. military aid to Ukraine "had become tied" to the investigations. 

"Everyone was in the loop," Sondland testified in opening remarks. "It was no secret." 

Sondland appeared prepared to fend off scrutiny over the way his testimony has shifted in closed-door settings, saying "my memory has not been perfect." He said the State Department left him without access to emails, call records and other documents he needed in the inquiry. 

Still, he did produce new emails and text messages to bolster his assertion that others in the administration were aware of the investigations he was pursuing for Trump from Ukraine.