A U.S. senator alleges that a Ukrainian oligarch and Paul Manafort associate who has been linked to Russian organized crime has made "hundreds of millions" in "illicit profits" while fighting extradition to the U.S.

Sen. Roger Wicker, R-Miss., wrote in an April 2018 letter obtained by NBC News that Dmytro Firtash had served as a "direct agent of the Kremlin" and was using the money earned from ongoing "corruption" to delay his extradition to Chicago, where he has been under federal indictment since 2014.

"This corruption undermines Ukrainian reform efforts that the United States strongly supports," Wicker wrote to then-Attorney General Jeff Sessions. He asked Sessions about the status of Firtash's extradition case.

The contents of Wicker's letter, which was obtained by NBC News after Freedom of Information Act litigation with the Justice Department, have not previously been reported.

Firtash was indicted in 2014 for what federal prosecutors in the Northern District of Illinois allege was his role in bribing Indian officials in order to get a lucrative mining deal to sell titanium to Boeing. He was arrested in Vienna in March 2014, released on $174 million bail, and has been contesting his extradition to the U.S. ever since.

Federal prosecutors said in a 2017 filing that Firtash and his co-defendant in the alleged scheme, Andras Knopp, "have been identified by United States law enforcement as two upper-echelon associates of Russian organized crime."

Wicker's letter alleges that prior to 2014, Firtash acted as a "direct agent of the Kremlin" during a separate scheme to skim money from natural gas transfers between Russia's Gazprom and Ukraine.

Court documents and leaked Department of State cables reviewed by NBC News say Firtash was involved in the scheme from at least 2005 to 2010. "By manipulating natural gas prices," the Wicker letter says, "the Kremlin used RosUkrEnergo as a tool to keep [Kiev] dependent on Moscow." According to the court documents, RosUkrEnergo was a firm created by Firtash and Gazprom, which is majority owned by the Russian government, to skim money.

"Despite his arrest" in 2014, Wicker wrote, "Firtash continues to engage in corruption in Ukraine, reaping hundreds of millions of dollars in illicit profits." His letter estimated the gross amount earned since Firtash's arrest at $1.5 billion.

Spokespersons and attorneys for Firtash, Lanny Davis and Dan Webb, said that the allegations in Wicker's letter were "categorically and unequivocally false."

"Every single one is a lie," Davis said. "There are no facts to support them."

Read the Davis statement here.

Davis, a former Bill Clinton aide who also serves as a spokesperson for former Trump attorney Michael Cohen, threatened NBC News with a defamation lawsuit if the contents of the Wicker letter were published.

"It appears that Sen. Wicker wrote the letter based upon misinformation," Davis said, "possibly using language similar to that used by a biased top manager of a large Ukrainian state-owned company who might be interested in building a vertically integrated monopoly in [the] gas business."

"This individual[,] possibly associated with other political operatives and defeated politicians in the Ukraine with similar biases, might have previously used U.S. media and political figures to spread lies about Mr. Firtash while trying to remain anonymous."

In a statement, Rick VanMeter, a spokesperson for Wicker, said, "As the co-chair of the Commission on Security and Cooperation in Europe, Senator Wicker regularly meets with foreign delegations. He has a deep interest in Ukraine and a strong record of opposing Vladimir Putin's aggression in the region. He stands firmly with the people of Ukraine fighting for freedom and reform. The letter was an inquiry from Senator Wicker to the U.S. administration seeking updates on efforts to extradite Mr. Firtash — widely known for his corruption and connection with the Kremlin and organized crime. The letter was written after considering a range of credible sources, including meetings with foreign delegations, publicly available data, and non-public information."

The NBC News FOIA litigation with the Justice Department did not produce any documents that show whether or not the Justice Department responded to Wicker.

A spokesperson for the U.S. Attorney's Office in Chicago declined to comment, citing the ongoing extradition proceedings.

"He was adamant that he had not committed a single crime"

Firtash, 54, owns a valuable portfolio of chemical, real estate and energy businesses. He is perhaps best known in the U.S. as a one-time associate of former Trump campaign chair Paul Manafort, who is now in federal prison.

In 2008, according to court records, Manafort's firm was involved with Firtash in a plan to redevelop New York City's Drake Hotel for $850 million. Firtash's company planned to invest more than $100 million, the records say.

One of the other partners working with Manafort on the deal was Brad Zackson, the former exclusive broker for the properties of President Donald Trump's late father, Fred Trump.

Eventually, documents show, Firtash's investment company transferred $25 million into escrow to further the project. The project was ultimately abandoned.

Davis wrote to NBC News after this story was published to dispute the escrow funding. "The assertion that Mr. Firtash or one of his companies transferred $25 million to an escrow account or to anyone in the U.S. regarding a NYC real estate deal or any deal is false." He said that it had been denied publicly many times, and "any assertion by any document to the contrary is false."

The document, which NBC News obtained as part of a public court filing, is on Firtash’s company’s letterhead, from the CEO of the real estate group, stating he is sending the letter on behalf of Firtash’s company. The document says that "as a demonstration of our commitment to this effort, Group DF has executed a deposit of twenty-five million dollars (US $25,000,000.00) into escrow for the project. Separately, we have executed and sent to you the 'Escrow Agreement' with First American Title Insurance Company documenting the escrow letter."

The letter was sent to Paul Manafort in November 2008 and can be read by clicking here.

Firtash was a major backer of Ukraine's Party of Regions, the pro-Russia party for which Manafort as a political consultant worked for many years, according to the federal criminal complaint against Firtash and a February 2006 State Department cable posted by WikiLeaks in 2010.

Ukrainian oligarch Dmytro Firtash waits for the start of his trial at the main court in Vienna, Austria,on April 30, 2015. Ronald Zak / AP file

Manafort's firm made more than $17 million in gross revenue from the party, according to his Foreign Agent Registration Act filing.

Another leaked cable said that Manafort's job in 2006 was to give the Party of Regions an "extreme makeover" and "change its image from … a haven for mobsters into that of a legitimate political party."

In previous interviews and statements to NBC News, Manafort has said he "never had a business relationship" with Firtash. "There was one occasion where an opportunity was explored. ... Nothing transpired and no business relationship was ever implemented."

According to a December 2008 State Department cable posted by WikiLeaks, Firtash told U.S. Ambassador William Taylor during a meeting at the U.S. Embassy in Kiev that he got his start in business with the permission of one of Russia's most well-known organized crime bosses, Semion Mogilevich.

Mogilevich was once on the FBI's top 10 most wanted list and has an Interpol Red Notice out for his arrest. The FBI alleges that he is "involved in weapons trafficking, contract murders, extortion, drug trafficking, and prostitution on an international scale."

According to the cable, Firtash told Taylor he was forced to deal with such people.

"[Firtash] was adamant that he had not committed a single crime when building his business empire," the cable says, and "argued that outsiders still failed to understand the period of lawlessness that reigned in Ukraine after the collapse of the Soviet Union."

At one point during the meeting, Firtash began to talk about "mistakes he might have made," the cable says, but was waved off by an accompanying adviser.

Davis said that "the assertion that according to a U.S. State Department cable, Mr. Firtash stated that he had to receive permission and approval from Mogilevich before proceeding with a Russian gas deal, is false. Mr. Firtash has already denied publicly this false statement many times."