(Reuters) - U.S. diplomat Gordon Sondland testified on Wednesday that he had informed U.S. Energy Secretary Rick Perry in July that Ukraine’s president would comply with President Donald Trump’s request for a probe of the Ukrainian gas company Burisma, where Joe Biden’s son was a board member.

FILE PHOTO: U.S. Secretary of Energy Rick Perry listens during a news conference after the Partnership for Transatlantic Energy Cooperation conference in Vilnius, Lithuania October 7, 2019. REUTERS/Ints Kalnins/File Photo

Sondland’s testimony, in a Democrat-led impeachment probe of Trump’s Ukraine dealings, raises questions about Perry, who had been one of Trump’s longest-serving and most scandal-free Cabinet members. Perry has denied being in any conversations where the Bidens were mentioned. Last month, Perry said he would step down from his post on Dec. 1.

The U.S. House impeachment investigation centers on Trump's request here during a July phone call with Ukraine President Volodymyr Zelenskiy that Kiev investigate the son of his Democratic rival and former Vice President Joe Biden.

Trump held up nearly $400 million in military aid to Ukraine as he asked Zelenskiy to investigate the Bidens.

Sondland, the U.S. ambassador to the European Union, testified that he sent Perry and other Trump officials an email a week before the July 25 call that said Zelenskiy “intends to run a fully transparent investigation and will ‘turn over every stone.’”

Here’s why Perry’s name keeps coming up in the U.S. House of Representatives investigation.

WHAT IS PERRY’S HISTORY IN UKRAINE?

Perry has been involved in Ukraine in his capacity as Trump’s top energy official since 2017, when Ukrainian President Petro Poroshenko was in power. That year, Perry helped seal a deal to sell U.S. coal to Ukraine as Kiev struggled to get fuel from pro-Russia regions.

Perry led a U.S. delegation to Zelenskiy’s inauguration in May as a last-minute substitute for Vice President Mike Pence. Democratic lawmakers want to know what kind of conversations he had with Ukraine’s newly elected leader.

Perry has been touting U.S. exports of what he calls “freedom gas” - a term he coined for American liquefied natural gas - that could provide Ukraine an alternative to Russian fuel.

WHO ARE ‘THE THREE AMIGOS’?

After briefing Trump on the Zelenskiy inauguration, Perry, Sondland, and Kurt Volker, the former U.S. envoy to Ukraine, became known as the “three amigos.” They were responsible for a side-channel Ukraine policy, under Rudy Giuliani, Trump’s personal lawyer.

Giuliani, at Trump’s behest, pushed Kiev to investigate Joe Biden, a contender for the Democratic presidential nomination next year, and his son Hunter, who was on the board of the Ukrainian gas company Burisma. The Bidens have not been accused of wrongdoing.

Sondland said on Wednesday Giuliani conveyed to Perry, Volker, and others that Trump wanted a public statement from Zelenskiy committing to investigations of Burisma and the 2016 election.

Sondland said in earlier testimony that he first connected Burisma with the Bidens after Perry and Volker told him about unspecified conditions that had to be met prior to the White House’s granting Zelenskiy a meeting with Trump. It has not come up in the testimony whether Perry had directly told Sondland that Trump wanted a probe of the Bidens.

HOW HAS PERRY RESPONDED?

Perry has said he has never had a meeting with Ukrainian or U.S. officials that mentioned Biden, or his son Hunter. “Not once, as God is my witness, not once was a Biden name, not the former vice president, not his son ever mentioned,” Perry told the Christian network CBN news in early October.

But according to Sondland’s testimony on Wednesday, Perry replied to Sondland’s email ahead of the July 25 call that mentioned Zelenskiy’s interest in the investigation that Trump sought, and said he had heard that the call between the two presidents was being arranged.

Perry has said he is stepping down Dec. 1, though news reports started appearing in April that he wanted to resign.

Perry has refused to hand over documents to investigators, or testify. A spokeswoman said he would not participate in a “secret star chamber inquisition.” Brian McCormack, Perry’s former chief of staff, and Wells Griffith, a National Security Council official on energy, also refused to testify.

The Energy Department’s spokeswoman, Shaylyn Hynes, said on Wednesday Sondland had misrepresented both Perry’s interaction with Giuliani and the direction he received from Trump. Perry spoke to Giuliani only once at Trump’s request, the DOE said. “No one else was on that call. At no point before, during or after that phone call did the words ‘Biden’ or ‘Burisma’ ever come up in the presence of Secretary Perry,” Hynes said.

Trump has said he made his July 25 call with Zelenskiy at Perry’s urging. Perry has said he merely wanted Trump to talk about liquefied natural gas.