Kurt D. Volker has resigned as the Trump administration’s special envoy for Ukraine, a person with knowledge of the event said Saturday. He is the first casualty of Congress’s impeachment inquiry into President Trump’s conduct with that country.

Volker tendered his resignation to Secretary of State Mike Pompeo on Friday, within hours of an announcement from three House committees that the veteran diplomat was among State Department officials who would be compelled to testify.

Those committees have set a Thursday hearing for Volker, one of several officials whose interactions with Trump or Ukrainian officials are outlined in a whistleblower complaint made public this past week.

The impeachment inquiry will examine whether Trump abused his office to lean on Ukraine’s leader to investigate Trump’s political rivals, and whether the White House tried to cover it up. House Democrats hope to conclude their work by the end of the year.

The Fact Checker unravels what happened when Trump tried to force an investigation into the false rumor about then-Vice President Joe Biden and Ukraine. (The Washington Post)

The State Department has not commented on Volker’s resignation, which was first reported by the State Press, Arizona State University’s student newspaper. The departure was confirmed by a person familiar with the events who spoke on the condition of anonymity to discuss a personnel matter.

Volker, a former career diplomat who heads the McCain Institute at ASU, had held the Ukraine job part time for the past two years.

He worked for months to facilitate a meeting between Trump and Ukrainian President Volodymyr Zelensky, the young anti-corruption reformer elected in April. That meeting may have been held up as part of Trump’s pressure campaign.

The committees are expected to examine Volker’s role in facilitating contacts between Trump’s personal attorney, Rudolph W. Giuliani, and officials of the new Ukrainian government this past summer. Those efforts appear to have been aimed at separating Giuliani’s interest in investigating former vice president Joe Biden and his son Hunter from official U.S. dealings with the Ukrainian government.

Volker was angered by what he saw as Giuliani’s attempts to interfere in the U.S.-Ukraine relationship by urging the newly elected Zelensky to focus on alleged corruption by Democrats, said a colleague familiar with Volker’s thinking.

Volker also was concerned that the president was going along with Giuliani’s agenda, said the colleague, who spoke on the condition of anonymity to be candid.

“He was very worried that Giuliani was screwing up the relationship,” the person said. “Here was a new president with a new mandate and he needed help from the West, particularly the United States. And what was Trump doing? Sending Giuliani around. And Kurt felt undermined and outraged.”

Volker was apparently not on a July telephone call between Trump and Zelensky that is at the center of the impeachment inquiry announced by House Speaker Nancy Pelosi (D-Calif.) this past week.

But Volker could be a significant figure in the inquiry because of his potential knowledge of Giuliani’s efforts, which Giuliani has claimed without corroboration had come at the behest of the State Department. If Volker was concerned about Giuliani and how he could shield Ukraine from any damaging interactions with the president’s private attorney, House Democrats will want to know.

Volker, according to the complaint, traveled to Kiev the day after Trump’s July 25 call to Zelensky. He was accompanied by Gordon Sondland, the U.S. ambassador to the European Union and a Trump ally.

“Based on multiple readouts of these meetings recounted to me by various U.S. officials, Ambassadors Volker and Sondland reportedly provided advice to the Ukrainian leadership about how to ‘navigate’ the demands that the President had made of Mr. Zelensky,” the whistleblower wrote.

Giuliani has long been a proponent of unsubstantiated theories that Biden’s son Hunter profited improperly from work in Ukraine and that as vice president, Joe Biden helped wave off a Ukraine corruption investigation that could have ensnared his son.

Hunter Biden served for nearly five years on the board of Burisma, Ukraine’s largest private gas company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment. Hunter Biden was not accused of any wrongdoing in the investigation. As vice president, Joe Biden pressured Ukraine to fire the top prosecutor, Viktor Shokin, who Biden and other Western officials said was not sufficiently pursuing corruption cases. At the time, the investigation into Burisma was dormant, according to former Ukrainian and U.S. officials.

There is also no evidence to substantiate the separate claim aired by Trump this past week that officials in Ukraine had helped his 2016 Democratic rival Hillary Clinton or that deleted emails from her time as secretary of state might reside in Ukraine.

Giuliani has published images of messages he exchanged with Volker to refute the notion that he was freelancing.

After a brief stint as a CIA analyst, Volker joined the State Department in 1988 and worked largely on European issues. He rose to become European director at the National Security Council under President George W. Bush, whom he served for less than a year as U.S. ambassador to NATO before being replaced after Barack Obama’s election.

Volker kept his job as executive director of the McCain Institute, dedicated to developing “character-driven leaders” in memory of the late senator John McCain (R-Ariz.), after he was tapped in 2017 as Ukraine envoy by then-Secretary of State Rex Tillerson.

Ellen Nakashima contributed to this report.