(Reuters) - U.S. Energy Secretary Rick Perry, who has built a reputation as one of President Donald Trump’s longest-serving and most scandal-free Cabinet members, has emerged as a source of interest in the Trump impeachment probe. Perry last month said he would step down from his post on Dec. 1.

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

The investigation centers on Trump's request here during a July phone call with Ukraine President Volodymyr Zelenskiy that Kiev investigate Democratic rival Joe Biden's son.

Here’s why Perry’s name keeps coming up in the U.S. House of Representatives investigation, and some of the questions that Democratic lawmakers will be looking to answer as they open public hearings on the probe next week.

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 attended Zelenskiy’s inauguration as Ukraine’s president in May this year, as a last-minute substitute for Vice President Mike Pence. Democratic lawmakers in the impeachment probe 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, U.S. Ambassador to the EU Gordon 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 for Kiev to investigate Biden, the former vice president and a contender for the Democratic presidential nomination next year, and his son Hunter, who was on the board of Ukrainian gas company Burisma, but it is uncertain what Perry knew about it.

-- In testimony released Tuesday Sondland told House lawmakers he first heard that Trump wanted a probe into Burisma from Volker, Perry and his former chief of staff Brian McCormack, who had all been in touch with Giuliani. Perry's name came up in Sondland's testimony 114 times here.

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

-- Sondland testified that Perry “volunteered to make the first outreach” to Giuliani about Ukraine policy because he was the most familiar with him. Lawmakers want to know what Giuliani told Perry in that outreach.

WHAT HAS PERRY SAID?

-- Perry has said he has never had a meeting with Ukrainian officials or U.S. officials that mentioned Biden, or his son Hunter.

-- Perry has refused to hand over documents to investigators, or testify. A spokeswoman said he would not participate in a “secret star chamber inquisition.” McCormack and Wells Griffith, a National Security Council official on energy, also refused to testify this week.

-- Trump said he made his July 25 call with Zelenskiy at Perry’s urging. A summary of the call showed that Trump held up nearly $400 million in military aid to Ukraine as he asked Zelenskiy to dig up dirt on the Bidens. Perry has said he merely wanted Trump to talk about liquefied natural gas.

-- Perry has said he is stepping down Dec. 1, though news reports based on sources that he wanted to step down had started appearing in April.

WHAT ELSE ARE INVESTIGATORS LOOKING AT?

-- Sondland met with Ukrainian officials on July 10 at the White House where he brought up Burisma during negotiations about granting Zelenskiy a meeting with Trump. An NBC News report said Fiona Hill, a former White House official, said she walked into the meeting as Perry was leaving. Lawmakers will ask what Perry heard during that meeting.

-- Sondland said in the testimony released this week that he spoke with Perry shortly before he testified. Lawmakers will likely ask whether the officials sought to get their stories straight before Sondland’s testimony.