WASHINGTON (Reuters) - The U.S. officials who will testify next week at public hearings in the House of Representatives impeachment probe against President Donald Trump include the current top diplomat in Ukraine, the previous ambassador there and a diplomat who has spent much of his career fighting corruption in Ukraine and elsewhere.

FILE PHOTO: Former U.S. ambassador to Ukraine Marie Yovanovitch arrives to testify in the U.S. House of Representatives impeachment inquiry into U.S. President Trump on Capitol Hill in Washington, U.S., October 11, 2019. REUTERS/Jonathan Ernst

All three officials have already offered testimony behind closed doors in the impeachment inquiry. They raised alarm bells about a White House efforts to pressure Ukraine to carry out politically motivated investigations that Trump, a Republican, had demanded.

Next week’s hearings will be the first to be held in public. Here are some details about the officials who will testify.

WILLIAM TAYLOR

The top U.S. diplomat in Ukraine, he provided some of the most important testimony in the impeachment inquiry so far. On Oct. 22 he said he had been concerned to learn that security aid to Ukraine, as well as a White House meeting between Trump and Ukrainian President Volodymyr Zelenskiy, had been delayed for political reasons.

In his testimony, Taylor said by mid-July “it was becoming clear to me that the meeting President Zelenskiy wanted was conditioned on the investigations of Burisma and alleged Ukrainian interference in the 2016 U.S. elections.”

Burisma is a Ukrainian company that paid Hunter Biden, the son of Democratic presidential candidate Joe Biden, to serve on its board. The allegation that Ukraine, not Russia, meddled in the 2016 elections is a discredited conspiracy theory that Trump sought to pursue.

Taylor also expressed his concerns in text messages released by House Democratic investigators, at one point texting that “it’s crazy to withhold security assistance for help with a political campaign.”

Trump has contended that there was no quid pro quo related to the $391 million to help Ukraine fight Russia-backed separatists in eastern Ukraine that he had withheld.

Taylor testified that he had been told that U.S. ambassador to the Europe Union, Gordon Sondland, told a Ukrainian official that the security aid would not arrive unless Zelenskiy publicly committed to the Burisma investigation Trump sought. Taylor’s testimony helped Sondland to “refresh” his memory, and he filed an amended statement with the committee in which he admitted making the comment to the Ukrainian.

Taylor will testify on Nov. 13.

MARIE YOVANOVITCH

Former U.S. ambassador to Ukraine Marie Yovanovitch, who was abruptly recalled from her post in May, told the impeachment inquiry on Oct. 11 that Trump ousted her based on “unfounded and false claims” after she had come under attack by his personal lawyer, Rudy Giuliani.

Yovanovitch, who has worked under both Republican and Democratic administrations, said Giuliani’s associates “may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine.”

Trump called Yovanovitch “bad news” in a phone call to Zelenskiy, according to a White House summary.

Yovanovitch also described how Sondland, the U.S. ambassador to the European Union, had urged her to use Twitter to express support for Trump in order to save her job. “He said, you know, you need to go big or go home. You need to, you know, tweet out there that you support the president,” she said.

Yovanovitch will testify in public on Nov. 15.

GEORGE KENT

George Kent, a senior U.S. diplomat, said in closed-door testimony Oct. 15 that he had been alarmed by efforts by Giuliani and others to pressure Ukraine, and detailed the people Giuliani relied upon for information.

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.

Kent is a diplomat who has spent much of his career fighting corruption in Ukraine and elsewhere.

Kent testified that a Biden staffer rebuffed him in 2015 when he raised concerns that his son’s involvement in Burisma involved a possible conflict of interest, according to a source familiar with his testimony. Kent will testify in public on Nov. 13.