Congressional Democrats are looking into whether a Ukraine natural gas oligarch with ties to organized crime provided funding to two associates of Rudy Giuliani who are now charged with making illegal campaign contributions to President Donald Trump and GOP lawmakers, Rep. Jamie Raskin said Wednesday.

Dmytro Firtash, a wealthy Ukrainian accused of skimming money from the country's state-owned natural gas company and who is fighting extradition to stand trial in the United States over bribery charges, is coming under increasing scrutiny as a possible source of the money that Lev Parnas and Igor Fruman, two Americans from Eastern Europe, are charged with illegally funneling to the Trump campaign and GOP lawmakers. Parnas and Fruman were indicted earlier this month, and they entered not guilty pleas in court on Wednesday, POLITICO wrote.

The ultimate source of their money is still not known. The indictment lists only "foreign national-1," a Russian, as providing funds for donations that Parnas and Fruman made between June 2018 and April 2019, including a $325,000 contribution to a Trump campaign PAC. That donation, as well as a $15,000 contribution to a PAC supporting West Virginia Attorney General Patrick Morrisey, was made under the name of a natural gas export company that Parnas and Fruman founded but that appears never to have conducted any business.

"There is growing interest in Mr. Firtash and what role he played," said Raskin, a member of the House Committee on Oversight and Reform that is one of the committees leading the impeachment inquiry into President Donald Trump. "His connection to Vladimir Putin and Russia is also one that needs to be explored. It's certainly being talked about among various members."

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Firtash is a Ukrainian citizen whom the Justice Department has tied to the Russian mafia, a relationship he has denied. Sen. Roger Wicker in April 2018 accused Firtash of being a "direct agent" of Russian President Vladimir Putin's government and using illicit money to fight extradition from his home in Vienna, Austria, to face trial in the bribery case, according to a letter Wicker sent to then-Attorney General Jeff Sessions and obtained by NBC earlier this year.

Wicker declined last week to comment about Firtash.

Firtash hired conservative lawyers and GOP operatives Joe diGenova and his wife Victoria Toensing to represent him in his fight against the Justice department. A spokesperson for the diGenova & Toensing law firm said Parnas, a Ukrainian-American who also has ties to organized crime and is a client of Giuliani, was hired as a translator for Firtash and his lawyers.

When asked whether Firtash had used Parnas, Fruman or their firm to supply money to the Trump or other Republican's campaign, the spokesperson replied "absolutely not."

Firtash owns many of Ukraine's regional natural gas distribution companies, which buy gas from the state-owned company Naftogaz and then resell it, sources close to the company and media reports have said. He is said to have resold an estimated $2 billion of natural gas provided by Naftogaz without reimbursing the state-run company, people close to the Ukraine government and gas industry told POLITICO.

"This is his cash cow," one person who works for the Ukraine government said of Firtash's connections to Naftogaz. If Firtash had been providing money to Parnas and Fruman, the person added, "the money from the gas corruption scheme in Ukraine is in the U.S. political system."

Parnas and Fruman also brought up Firtash's name to Ukrainian officials at a Houston meeting in March as they pitched their company Global Energy Producers LLC as a supplier of U.S. gas, according to Dale Perry, an executive at a U.S. gas company who does business in Ukraine and was informed about the meeting. NBC News first reported the two men's invoking of Firtash in their business dealings.

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Firtash also pressured the administration of former Ukrainian President Petro Poroshenko to remove Naftogaz's reform-minded chief executive, Andriy Kobolyev, Perry said. Kobolyev had the backing of former U.S. ambassador to Ukraine Marie Yovanovitch to make those reforms.

Firtash may have had the motive to help Parnas and Fruman push for Yovanovitch's removal, Perry told POLITICO.

"Firtash has the money and he has the reasons," Perry said.

Parnas and Fruman also donated money to former Rep. Pete Sessions, who advised that Trump recall Yovanovitch to the U.S. after the donation. Sessions told POLITICO earlier that he had met with Parnas and Fruman but declined to say whether he was the lawmaker their indictment named as "Congressman 1."

Koboylev did not respond to POLITICO's emails seeking comment. John Dowd, the lawyer representing Parnas and Fruman, did not respond to multiple calls and emails.