(Reuters) - Former U.S. Ambassador to NATO Kurt Volker on Thursday became the first of five former and current U.S. diplomats to give a deposition to the congressional committees conducting an impeachment inquiry into U.S. President Donald Trump.

Volker resigned last week as Trump’s special representative for Ukraine negotiations.

Democratic lawmakers are examining whether there are grounds to impeach Republican Trump based on a whistleblower’s account that said he asked Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to help investigate Democratic political rival Joe Biden.

The following are descriptions of Volker and four serving U.S. diplomats whom lawmakers have asked to give sworn, closed-door testimony:

KURT VOLKER

Volker left the foreign service a decade ago after a meteoric career that made him the second-ranking U.S. diplomat for Europe at 40 and the U.S. ambassador to the NATO Western security alliance at 43.

Current and former U.S. officials described Volker, whose jobs including serving as an aide to the secretary-general of the North Atlantic Treaty Organization and as a director on the White House national security council, as a talented diplomat.

They said his rapid rise in George W. Bush’s administration, when he vaulted over older colleagues to become the number two official in the Bureau of European and Eurasian Affairs, may have led to his decision to leave the foreign service.

Having spent only about a year in what one current official called his “dream job” as U.S. ambassador to NATO, Volker was replaced by former President Barack Obama and may have felt there was little reason to stay.

“He was ... an extremely capable officer who had very good grasp of U.S. foreign policy - especially for the big picture, not just the thing that he was working on,” one official said.

After working in the private sector, Volker became head of the McCain Institute for International Leadership at Arizona State University. In 2017, he took an unpaid role as the U.S. special representative for Ukraine negotiations.

Volker is cited in the whistleblower report as having met Zelenskiy one day after the July 25 call, along with U.S. Ambassador to the European Union Gordon Sondland, and having provided advice about how to “navigate” Trump’s demands.

The report also said Volker and Sondland had spoken to Trump’s personal lawyer Rudy Giuliani to try to “contain the damage” from what some officials saw as the lawyer’s end run around normal policy-making.

Giuliani has acknowledged meeting Ukrainian officials in Madrid, Paris and Warsaw this year as he pushed an investigation into Biden, a potential Democratic rival of Trump’s in the 2020 presidential election. However, he has told Reuters he did not view his work as a “political act” and said it began in 2018, before Biden had become a candidate.

The McCain Institute declined comment.

Kurt Volker, U.S. President Donald Trump's former envoy to Ukraine, arrives to be interviewed by staff for three House of Representatives committees as part of the impeachment inquiry into the president's dealings with Ukraine, at the U.S. Capitol in Washington, U.S. October 3, 2019. REUTERS/Jonathan Ernst

Volker is one of two of the five current and former diplomats who have agreed to speak to the committees, congressional aides said. The other is a former U.S. Ambassador to Ukraine.

MARIE “MASHA” YOVANOVITCH

A three-time U.S. ambassador who has led the U.S. embassies in Armenia, Kyrgyzstan and Ukraine, Yovanovitch’s stint as U.S. ambassador in Kiev was cut short in May, when she was brought back to Washington under murky circumstances.

Described by colleagues as a professional to her fingertips, Yovanovitch became the subject of allegations - flatly denied by the State Department - that she gave a Ukrainian prosecutor a list of people not to prosecute.

Yovanovitch, a career diplomat, has been accused by Trump allies of criticizing the president to foreign officials, something current and former colleagues found inconceivable.

“There is zero possibility, absolutely zero possibility, that happened. It cannot possibly be true,” said a current U.S. official who has known Yovanovitch for decades.

When Yovanovitch was brought back to the United States, the State Department said she was “concluding her 3-year diplomatic assignment in Kiev in 2019 as planned” and that her departure date “aligns with the presidential transition in Ukraine.”

Current and former officials, however, said that Yovanovitch had not been expected to leave Kiev until later in the year and said a presidential transition is precisely when you want an experienced ambassador in place.

Yovanovitch agreed to give a deposition on Oct. 11, committee officials said. She could not be reached for comment and the State Department did not respond to requests for comment.

GEORGE KENT

Kent, who majored in Russian Language and Literature as an undergraduate at Harvard, is a deputy assistant secretary of state overseeing U.S. policy toward Ukraine, Moldova, Belarus, Georgia, Armenia, and Azerbaijan.

The most junior of the U.S. diplomats who has been asked to address the committees, Kent is a career foreign service officer who has served in Warsaw, Kiev, Tashkent and Bangkok.

He has also worked as a special assistant to the State Department’s number three official, a demanding but plum job that typically goes to diplomats who are rising fast and often become ambassadors.

Kent, a former official said, is steeped in the history and politics of Ukraine.

“I don’t think there is anybody in the U.S. government who has a better understanding of the current politics,” said the former official. “He is the kind of foreign service officer who goes abroad and ... immerses himself in the foreign culture.”

T. ULRICH BRECHBUHL

Brechbuhl, counselor to the secretary of state, has ties to U.S. Secretary of State Mike Pompeo going back more than three decades to their time as classmates at the U.S. Military Academy at West Point.

Brechbuhl then followed Pompeo into business and to the State Department.

Both graduated from West Point in 1986 and Brechbuhl, according to his State Department biography, went on to serve as a cavalry officer patrolling the Iron Curtain before its fall and in the First Calvary Division during the Persian Gulf War.

After leaving active duty, Brechbuhl received a Masters in Business Administration from Harvard, worked at Bain and Company Inc and joined Pompeo in founding a Kansas-based aerospace company.

“I saw him as the guy who was going to protect Pompeo’s back,” said a former State Department official. However, the former official said he did not regard Brechbuhl - who was born in Switzerland and grew up in Garden City, New York - as “political or ideological.”

The whistleblower report said that Brechbuhl listened in on the July 25 call but a State Department official denied this.

GORDON SONDLAND

The founder and chief executive of Provenance Hotels, which own or operate 19 boutique hotels, most of which are in the Pacific Northwest, is a political appointee who serves as the U.S. ambassador to the European Union.

The Brussels-based post typically involves U.S. trade policy and dealing with the EU’s bureaucracy but Sondland has sought to expand its scope, including to such issues as Ukraine.

“Of the political appointees he is serious, he is there to work but he has the disadvantage of working for an administration that is deeply ... anti-EU,” said a former U.S. official.

“When I read the (whistleblower) account of what Volker and Sondland were trying to do with the Ukrainians and Giuliani, trying to limit the damage, I found that entirely plausible,” the former official added. “I might have done the same.”

The State Department did not respond to requests for comment about Yovanovitch, Kent and Sondland.