When the US expanded its sanctions on Thursday to include members of Vladimir Putin's inner circle, one name in particular jumped out: Gennady Timchenko, a long-time acquaintance of Putin who rose from relative obscurity to become one of Russia's richest men. Timchenko's Gunvor Group enjoyed a similarly meteoric rise and is now the fourth-largest oil-trading firm in the world.

In explaining the sanctions, the US treasury department claimed what others have long suspected. "Timchenko's activities in the energy sector have been directly linked to Putin. Putin has investments in Gunvor and may have access to Gunvor funds," it said.

Gunvor rushed to deny the Putin link, calling the allegations "outrageous" and saying: "President Putin has not and never has had any ownership, beneficial or otherwise, in Gunvor."

But the long personal ties between the two have raised many questions about the serendipity of Timchenko's rise. Like Arkady Rotenberg, another businessman on the sanctions list who has enjoyed astounding business success under Putin, Timchenko is an old judo buddy of Putin's, and they even reportedly founded a judo club together.

This year, Forbes estimated Timchenko's wealth at $15.3bn (£9.3bn), calling him "one of the most powerful people in Russia".

Putin has previously said that he has had nothing to do with the oligarch's business activities. At a meeting with writers and publishers in 2011, Putin said that although he had known Timchenko from when Putin worked in St Petersburg city hall in the 1990s, he "had never meddled" in Timchenko's business interests and did not plan to.

"Timchenko got into business … from the very start, when privatisation was allowed. And I assure you … it was absolutely without my participation," Putin said.

However, Timchenko and a committee headed by Putin had a short-lived business venture in the 1990s, and a firm headed by Timchenko gained from an oil-for-food scheme set up by Putin in 1991, the FT said.

Timchenko set up the oil-trading firm Gunvor Group in 2000, which held a small share of the market until 2003, when the breakup of Yukos, the oil firm of Putin's enemy Mikhail Khodorkovsky, offered a unique opportunity. Gunvor's revenues grew from $5bn in 2005 to $43bn in 2007, a feat that many doubt could have been accomplished without good political connections.

In 2007, the political analyst Stanislav Belkovsky, a former Kremlin insider who said the Russian takeover of Crimea had convinced him to move to Ukraine, said Putin indirectly controlled 75% of Gunvor, with 37% of Surgutneftgaz and 4.5% of Gazprom. Many, however, see the claims as extravagant: in all, Putin's net worth was at least $40bn, he said, which would make him one of the world's richest men.

According to Investcafe analyst Andrei Shenk, US treasury claims that Putin is a beneficiary of Gunvor are not very believable. "It's hard to imagine that Putin would directly own any stakes. "

"First, it would be almost impossible to prove this, and secondly, there is no need [to own shares], it would carry a risk but no especially good commercial benefit," he said. Nonetheless, Timchenko has been among the Putin-connected businessmen to profit from lucrative Olympic construction projects.

The opposition figure Alexei Navalny's Anti-Corruption Foundation gave Timchenko and his associates a silver medal in mock awards for the "winners of the Olympics before their start," reporting that their SK Most construction company received a contract to build part of the famously expensive motorway and railway between Sochi's mountain and coastal clusters of Olympic venues. Navalny estimated that SK Most received 180bn of the 264.2bn rubles ($8.6bn) spent on the project.

In response, Timchenko's Volga Group investment vehicle published a statement in which it confirmed that SK Most had conducted work on 20 miles of the ilometre highmotorway-and-railway project, but noted the costliness of building the scheme which included tunnels "that have no analogy in worldwide practice," in difficult mountainous conditions.

Timchenko was not wrongfooted by the US sanctions. Gunvor Group said in a statement today that he had sold his stake to co-founder Torbjorn Törnqvist. Shenk said it was believable that Timchenko had anticipated the sanctions, since "the circle of people against which sanctions could be taken was small".

"There was a high probability he would fall under these sanctions, and he managed to sell," Shenk said.

But the well-known Russian business publication Slon.ru raised the possibility that the deal had been backdated. Although Gunvor should not have been affected by sanctions, which are only applied to companies owned at least in half by a blacklisted individual, Timchenko may have sold his shares as a precaution, since the Treasury Department nonetheless recommends that American companies not do business with companies in which a blacklisted individual holds a smaller stake, it reported.

"Another question is whether suspicions will arise that such a speedy, possibly backdated, deal is a sham," Slon.ru wrote.

But even if Timchenko avoided losing his shirt when selling his Gunvor stake, US sanctions could hurt him in other places. According to the Volga Group website, as of June 2013, Timchenko owned 50% of Arena Events, the other half of which was owned by the blacklisted Rotenberg brothers. That same year, the company acquired Karl's Event Services, which has offices in New York and Chicago, Slon.ru reported, citing a company statement.

Furthermore, Timchenko is a Finnish citizen and lives in Switzerland, which means his life could get more difficult if his name is on the newly expanded EU sanctions list, which includes assets and travel bans.