A U.S. senator's office says that after 18 months it has still received no answer from the Justice Department about why a Ukrainian oligarch linked to Paul Manafort and two men who work with Rudolph Giuliani has not been extradited to the U.S. to face federal bribery charges.

A spokesperson for Sen. Roger Wicker, R-Miss., said, "We have not received a response" to a 2018 letter about Dmytro Firtash.

On June 25, Austria approved the extradition of Firtash, who has been living in Vienna since his 2014 arrest, but Firtash has asked that the case be reopened.

Sen. Wicker sent a letter to then-Attorney General Jeff Sessions in April 2018 asking about the status of Firtash's extradition. NBC News exclusively obtained that letter through Freedom of Information Act litigation with the Justice Department.

Wicker alleged in the letter that Firtash, who has also been linked to Russian organized crime, had made "hundreds of millions" in "illicit profits" while fighting extradition to the U.S.

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

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."

Rudolph Giuliani has denied any involvement with Firtash and says he has never met him. One of the two Ukrainian-American Giuliani associates who was arrested at Dulles airport before boarding a flight to Vienna last week worked for the Firtash legal team as a translator. Giuliani had planned a trip to Vienna at the same time but said he was not planning to meet with Firtash.

The Firtash legal team includes Joe diGenova and Victoria Toensing, the pro-Trump husband and wife attorneys who Fox News reported were "working off the books" with Giuliani as part of his Ukrainian venture, in which he was seeking information about Joe and Hunter Biden and about the Ukrainian gas industry.

"So, Firtash, I know nothing about," Giuliani told NBC News. "I'm not going to answer any questions about because I'm probably going to get it wrong, and you can ask them."

Giuliani said the two men arrested at Dulles on Oct. 9 for campaign finance violations, Lev Parnas and Igor Fruman, "help me find people," and that he also represents their company as a lawyer.

Parnas is employed by Firtash's legal team, according to Mark Corallo, spokesperson for diGenova and Toensing.

A 'direct agent of the Kremlin'

Sen. Wicker's 2018 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.

Corallo, the spokesperson, said in a statement, "To reiterate what co-counsel for Mr. Firtash have already stated to NBC: The allegations about Mr. Firtash in Senator Wicker’s letter are false, disparaging and defamatory. Senator Wicker wrote his letter based on misinformation likely generated by a competitor in the Ukrainian natural gas industry who would benefit from Mr. Firtash’s exit from the industry."

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."

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.

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

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

A leaked State Department able 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."