Pavel Fuchs, a Ukrainian oligarch who negotiated with Donald Trump and reportedly works with his personal lawyer Rudy Giuliani, stands accused of making a threat of violence against a US-based businessman involved in selling him seats to the 45th US president's inauguration.

The gas and real estate tycoon was furious that two tickets he purchased for $200,000 left him and a Ukrainian member of parliament with bad seats, according to two well-placed sources speaking to Al Jazeera's Investigative Unit, a Ukrainian newspaper editor and other journalists.

Fuchs allegedly warned the businessman who facilitated the purchase he wanted his money back and if he did not get it he would "tear him apart".

It's alleged he then issued a series of threats to others involved in the deal.

The mogul was featured in the Al Jazeera investigation The Oligarchs, which aired in January 2018 and revealed that he has had at least a 10-year relationship with Trump and his son, Don Jr.

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Then a leading Russian real-estate figure, he held talks with the Trumps between 2008 and 2010 about constructing a Trump Tower in Moscow, which was never developed.

The presence of at least a dozen Ukrainian politicians and business figures, some of whom have close links to Russia, at the January 20, 2017 inauguration attracted interest from US Special Counsel Robert Mueller, according to the New York Times.

The report said some of the Ukrainian guests were "promoting grand bargains or peace plans" aligned with the Kremlin's interests.

Mueller is investigating Russian interference in the 2016 election and the newspaper said US federal prosecutors have begun a number of other inquiries related to the Ukrainians at the inauguration.

Fuchs, who was among them, tried to re-enter the US in December 2017, but was detained and deported by US border officials who revoked his visa, according to several sources.

The oligarch, who owns property in New York and Miami, later claimed he had been the victim of an administrative error.

It is not clear why agents refused to admit the Ukrainian, who emigrated from Russia in 2015.

US officials were not immediately available for comment on whether their decision was related to the alleged threats or to his wider business activities, which have drawn the attention of Ukrainian authorities.

Just found another photo of Pavel Fuchs / Fuks with Rudy Giuliani from his Russian wikipedia page. Says they met in July 2017 in New York, most likely at RG’s office. https://t.co/p7s0maCM0V pic.twitter.com/N4gWYMKF08 — Will Jordan (@willjordan) January 29, 2018

Al Jazeera investigation

The year-long Al Jazeera investigation into Eastern European oligarchs described Fuchs's role in purchasing $160m of frozen assets deemed to have been stolen from the Ukrainian state.

As Al Jazeera reported at the time, the evidence suggested the seller was a fugitive Ukrainian businessman close to the pro-Russian former President Viktor Yanukovych.

The businessman and the president were among those who fled at the height of Ukraine's Euro Maidan revolution in 2014, with Yanukovych now widely reported to be under Russian government protection.

More recently, Fuchs has been photographed with Giuliani, Trump's personal lawyer, in both New York and Ukraine's second city of Kharkiv.

The former New York mayor's security consulting business, Giuliani Security and Safety, is providing advice on improving Kharkiv's emergency services.

Kharkiv's residents are divided between those who are pro-Western and those who support the Russian government. Four years ago, the city nearly turned into a Russian-backed separatist enclave.

Fuchs is reported to be working directly with Giuliani to set up a Kharkiv investment office in the United States.

In mid-November last year, Fuchs was also allegedly refused entry to Israel.

In recent days, a Ukrainian politician reported the business figure had been detained and deported from Mexico, where he arrived by private jet with another legislator.

Ukraine media reports Fuchs now arrested in Mexico. Previously refused entry to Israel and US, per reports. Negotiated with Trump over Moscow tower 2008-10. Involved in shadow trade of frozen assets stolen by ex president Yanukovych. https://t.co/p1K1Ghy9SA — Will Jordan (@willjordan) January 8, 2019

The oligarch has refuted the story, posting a video of himself in what appears to be Mexico and giving the finger to the "fat pig" who first reported he had not entered the country.

"This is a fake," he said. "I'm in Mexico. Here are the Mexicans so there are no questions."

He later issued a statement in which he accused the person who first published the story, former lawmaker and businessman Mikhail Brodsky, of publishing "fake news".

"Perhaps this was done for the purpose of further extortion of money," he wrote.

'Mercenary'

Born in Kharkiv in 1971, Fuchs earned the nickname "Mercenary" from his associates, according to restricted Russian interior ministry records.

He built a network of businessmen close to ex-Moscow Mayor Yuri Luzhkov, who was later accused by Russian authorities of presiding over the loss of $5bn from city coffers.

The former mayor denied any wrongdoing and called the allegations "political bullying".

During Luzhkov's tenure, Fuchs made hundreds of millions of dollars from real estate and led the development of a new financial district which today dominates the skyline.

The Moscow City complex has been beset with financial difficulties since its main financier, Kazakh billionaire Mukhtar Ablyazov, was charged over an unrelated $6bn fraud case, which continues.

During a tour of one of his developments, Fuchs told Russian television that when he was young he beat people up. "I don't like it when someone lies to me."

He recounts a story about his Turkish builders. "While refurbishing, I told them not to smoke, but when I arrived they were smoking. I asked them again not to smoke."

"And they continued smoking?" asks the presenter.

"Yes, and they had to eat their cigarette butts," Fuchs replies.

"How beautiful," remarks the presenter, sarcastically.

Amid rumours of commercial disputes, the real estate tycoon fled Moscow in 2015 and resettled in Ukraine. He has now been placed under sanctions by the Russian government and cannot enter the country.

Despite a formal dispute with Russia, many in Ukraine suspect Fuchs of secretly operating with the consent of the Kremlin.

After the overthrow of Yanukovych, Ukraine's pro-Western government has been involved in a low-level, ongoing military conflict with Russia.

There are several enclaves in the east of the country that are controlled by Russian troops and Ukrainians who support the Kremlin.

Since Fuchs's arrival in the Ukrainian capital, Kiev, he has forged strong links with powerful politicians and accumulated assets previously owned by politicians and businessman close to Yanukovych.

He has become a leading figure in the gas industry, owning the second highest number of extraction permits, according to ANTAC, an anti-corruption campaign group.

In August, following the release of Al Jazeera's documentary, Fuchs was questioned by Ukrainian prosecutors.

Their case focused on whether he or others had been involved in "assisting the unidentified persons of a criminal organisation" in compromising the seizure of the $1.5bn deemed stolen by former president Yanukovych.

No charges have been brought so far.

Neither Fuchs nor a spokesperson for Giuliani responded to requests for comment.