WASHINGTON (Reuters) - The U.S. House of Representatives’ impeachment inquiry into whether President Donald Trump abused the power of his office to push the Ukrainian president to order an probe of a political rival has heard from a series of administration insiders over the past two weeks.

Kurt Volker, U.S. President Donald Trump's former envoy to Ukraine, arrives to participate in a closed-door deposition as part of the Democratic-led U.S. House of Representatives impeachment inquiry into U.S. President Donald Trump on Capitol Hill in Washington, U.S., October 16, 2019. REUTERS/Carlos Jasso

At the center of the inquiry is a July 25 call in which Trump pressed Ukrainian President Volodymyr Zekenskiy to investigate former vice president Joe Biden, a leading candidate for the 2020 Democratic presidential nomination, and his son Hunter Biden over their ties to a Ukrainian energy company.

Trump has acknowledged many of the central facts related to the call, maintaining that none of it amounted to wrongdoing.

Despite a White House declaration last week that the administration would not cooperate with an impeachment inquiry that it dismissed as a “kangaroo court,” the three Democratic-led committees leading the probe have heard from a series of witnesses.

Below are highlights of the testimony so far:

KURT VOLKER

The first witness, Kurt Volker, was the U.S. special representative for Ukraine until he resigned the day after the public release of a whistleblower’s complaint filed by a U.S. intelligence official that cited the call. The complaint described Volker as trying to mitigate the damage from efforts by Trump’s lawyer Rudy Giuliani to pressure Ukraine to investigate Democrats.

Volker’s Oct. 3 testimony revealed that he and Gordon Sondland, the U.S. ambassador to the European Union, drafted a statement for Zelenskiy that would have committed Ukraine to investigate the Bidens.

It also called for Ukraine to probe a discredited conservative conspiracy theory that maintained that Ukraine, not Russia, had meddled in the 2016 U.S. presidential election, a person familiar with the briefing said.

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MARIE YOVANOVITCH

The former U.S. ambassador to Ukraine, Yovanovitch was abruptly recalled from her post in Kiev after what she said was “a concerted campaign” against her, according to a copy of her Oct. 11 opening statement posted online by U.S. media.

Giuliani has accused Yovanovitch of blocking efforts to convince Ukraine to investigate the Bidens and suggested she was biased against Trump.

Trump also specifically referenced her in his July call with Zelenskiy, saying “the woman was bad news and the people she was dealing with in the Ukraine were bad news,” according to a White House summary of the call.

FIONA HILL

Fiona Hill, the former senior director for European and Russian affairs on Trump’s National Security Council, on Monday recounted for lawmakers a July 10 meeting that she attended with senior U.S. and Ukrainian officials, including Sondland, according to a person familiar with her testimony.

Hill said Sondland raised the matter of investigations, which she and others took as a reference to a probe into the Biden family, the person said.

She was alarmed at what she heard and was advised to see the lawyer for the National Security Council, according to the person.

GEORGE KENT

George Kent, a senior U.S. diplomat, on Tuesday said he had been alarmed by efforts by Giuliani and others to pressure Ukraine and detailed the people Giuliani relied upon for information, according to a lawmaker familiar with his testimony.

Kent also said a top White House official, acting chief of staff Mick Mulvaney, picked the officials responsible for Ukraine policy after Yovanovitch was recalled in May, Democratic Representative Gerry Connolly told reporters.

MICHAEL MCKINLEY

The former adviser to U.S. Secretary of State Mike Pompeo said on Wednesday told investigators he had resigned from his post over the State Department leadership’s unwillingness to defend Yovanovitch, according to people familiar with McKinley’s testimony.

GORDON SONDLAND

A hotelier and Trump political donor, Gordon Sondland told lawmakers that Trump directed senior officials to speak directly to Giuliani about U.S. policy in Ukraine.

Sondland said in his prepared Thursday remarks that he did not understand “until much later” that Giuliani was pushing for Ukraine to investigate Biden.

Sondland also participated in a series of text messages with Volker and another top U.S. diplomat, Bill Taylor. In the messages, he quoted the president when he wrote there was no quid pro quo, a Latin phrase meaning a favor in exchange for a favor, according to his prepared comments.