After hours of interviewing witnesses on Capitol Hill, House Democrats involved in the Ukraine investigation are still missing a crucial piece to the puzzle: how Energy Secretary Rick Perry fits into efforts to convince Ukraine to probe Joe and Hunter Biden, as well as the origins of Special Counsel Robert Mueller’s Russia investigation.

And a growing pressure campaign by the White House to push officials to defy subpoenas and steer clear of testifying could limit lawmakers’ ability to find the answers.

Perry’s name first surfaced in the Ukraine investigation when The Washington Post reported last month that President Trump said he took the now-infamous call with Ukrainian President Volodymyr Zelensky on July 25 because the secretary urged him to. Trump said Perry wanted him to speak with Zelensky about a liquefied natural gas (LNG) project. And witnesses interviewed by House investigators of the affair named Perry as one of the Trump administration’s main operators in Ukraine tackling U.S. foreign policy.

Yet little has been said about Perry’s interactions with officials in Zelensky’s administration or with associates of Trump’s personal lawyer Rudy Giuliani. And sources on Capitol Hill involved in the probe into efforts to convince Ukraine to investigate the Bidens and the Democratic National Committee say Perry’s entanglement in the story is still unclear.

Democrats were prepared to dig deeper on that looming question this week and scheduled depositions for both Perry and Brian McCormack, his former chief of staff. But Perry, who is set to step down from his position in the administration, refused to appear for closed-door questioning, saying he would consider complying with an open-hearing request. And two individuals with knowledge of the deposition schedule on the Hill said they are prepared for the possibility that McCormack does not appear for his testimony Monday. The Wall Street Journal reported early Monday that four Trump administration officials, including McCormack, were not expected to show up to testify, despite House Intelligence Committee subpoenas issued Sunday night. That could leave a gaping hole in the Ukraine timeline.

The Department of Energy did not immediately respond to a request for comment.

Two individuals involved in the House investigation told The Daily Beast that lawmakers are prepared to move into the public hearing phase regardless of whether witnesses appear for their depositions this week. At the same time, though, those sources said Democrats are eager to get more information about Perry’s communications about Ukraine with Trump, Giuliani, and his associates.

Investigators are interested in probing whether Perry’s responsibilities as energy secretary intersected with the Giuliani-Trump influence operation. Besides U.S. military aid for its war with Russia, Ukraine relies heavily on the United States for support in its energy sector and, like past administrations, the Zelensky team put the issue of natural-gas and energy independence at the top of its foreign-policy agenda.

Two former officials in the Trump White House told The Daily Beast that in general, the U.S. relationship with Ukraine is based on the understanding that the U.S. will try to prevent the construction of a Russian pipeline, also known as Nord Stream 2, in exchange for Ukraine fulfilling U.S. demands such as ridding the country of corruption.

“ It’s like everything is a quid pro quo with Ukraine. Ukraine relies on the U.S. for a lot, and we expect a lot in return. ” — Former White House official

“It’s like everything is a quid pro quo with Ukraine,” one former White House official said. “Ukraine relies on the U.S. for a lot, and we expect a lot in return.”

Nord Stream 2 is a Russian project that is expected to transport natural gas to Europe by bypassing Ukraine, severely limiting Kyiv’s access to a reliable energy source. The U.S. has opposed the idea of the pipeline for decades, but Trump administration officials, including U.S. Ambassador to the European Union Gordon Sondland, have made it a priority to publicly push back against the project in recent months. Meanwhile, Perry has pushed the idea of providing Ukraine and other Eastern European countries with U.S. liquefied natural gas.

Over the last six months, in the midst of a Giuliani-led campaign to oust U.S. Ambassador to Ukraine Marie Yovanovitch and to dig up dirt on Joe and Hunter Biden, Perry has focused his efforts primarily on Ukraine’s economy, its energy sector, and the ways in which the U.S. could participate in their development, according to four officials with knowledge of his conversations with Ukrainian officials. One source close to the Zelensky administration said Perry advocated for reform in the country’s gas sector as a way to attract additional American investment.

For the most part, Perry and his team at the Department of Energy steered clear of the president’s personal lawyer and his work in the country, according to two officials. But Perry did communicate with Giuliani, his associates, and Trump about Ukraine policy, corruption in the country, and a controversial energy proposal. And that’s what’s piqued Democrats’ interest.

The Department of Energy said Perry did in fact encourage Trump to speak with Zelensky about energy issues. Perry told Fox News in an interview that he had spoken with Trump about corruption in Ukraine in the past. But he insists the topic of the Bidens never came up in his conversations with the president or Ukrainian officials.

Still, Sondland said in his House testimony that he and Perry, as well as former top diplomat for Ukraine Kurt Volker, were the “three amigos” who coordinated Ukraine policy for the administration. Perry met with Zelensky several times, including on a trip to Kyiv with Sondland and Volker for the Ukrainian president’s inauguration. And Perry spoke with Giuliani, at the president’s direction, about setting up a meeting between Trump and Zelensky, according to The Wall Street Journal.

Of particular interest to House investigators are Perry’s efforts to replace members of the Ukrainian gas company Naftogaz’s board with a friendlier team. Perry pushed for the removal of Amos Hochstein, an American and a former Biden aide, as well as Bruno Lescoeur, a former executive at Engie, a French company, and Clare Spottiswoode, director of London-based advisory Gas Strategies, according to multiple news reports.

During the same time period, Giuliani’s associates were also working to replace members of the Naftogaz board. It is unclear if the plans overlapped.

Igor Fruman and Lev Parnas, both of whom were indicted by prosecutors in New York for allegedly violating campaign-finance laws, worked with Giuliani in Ukraine to get Ukrainian officials to investigate Hunter Biden, who served on the board of a large Ukrainian gas company, and the origins of Mueller’s Russia probe. Fruman and Parnas pushed Naftogaz to replace CEO Andriy Kobolyev, according to the Associated Press.

Investigators on Capitol Hill are also interested in the details of a pitch by Fruman and Parnas during their dealings with Naftogaz to ship U.S. liquefied natural gas to Ukraine, according to two sources familiar with the broader Ukraine probe. One of those sources said the investigating committees were particularly keen on asking questions about whether Perry was aware of that LNG plan and if he pitched it to Ukrainian officials in the Zelensky administration.

The U.S. cannot technically export its LNG to Ukraine directly because it cannot ship it through the Bosphorus in Turkey and because Ukraine does not have an import terminal to receive it, according to Anna Mikulska, an energy expert at Rice University.

Still, Perry has publicly spoken about the need for increased U.S. exports of LNG, particularly to Eastern Europe and Ukraine. At a conference in Brussels earlier this year, Perry touted American gas as “freedom gas.”

In an event in Lithuania last month, Perry spoke with Ukrainian and Polish officials about the idea of shipping U.S. LNG to Eastern Europe that could then be distributed to Ukraine. When asked by reporters about his attempt to overhaul the Naftogaz board as detailed in an investigation by the Associated Press, Perry said the report “was a totally dreamed-up story.”

—with additional reporting by Betsy Swan