MOSCOW (Reuters) - Russian law enforcement officers investigating bribery allegations on Monday detained two senior executives who work for billionaire Viktor Vekselberg, one of a dwindling group of tycoons who built their fortunes independently of the Kremlin.

Members of Russia's FSB security service are seen in the offices of Renova during a search of the offices of its subsidiary, T Plus, which also shares the premises of Russian tycoon Viktor Vekselberg's Renova group, in Moscow, Russia, September 5, 2016. REUTERS/Sergei Karpukhin

State investigators said they had opened a criminal case into allegations that executives in firms controlled by Vekselberg had bribed regional officials. Armed state security agents in ski masks searched the central Moscow offices of Renova Group, which manages Vekselberg’s assets.

“The company will cooperate with law-enforcement bodies. All staff are working as usual,” Renova spokesman Andrey Shtorkh said. “This is not connected with Vekselberg personally.”

Vekselberg, who is listed by Forbes magazine as Russia’s 7th richest person, could not be reached for comment.

Vekselberg’s conglomerate has interests in telecoms, mining, and utilities. The tycoon, 59, once won favor with the Kremlin by buying a set of Faberge eggs made for the former Russian royal family and bringing them back to Russia.

But as President Vladimir Putin has tightened the state’s control over all aspects of the economy, Vekselberg and tycoons like him who are not part of Putin’s inner circle have seen their influence wane.

After several criminal inquiries were launched this year into people in Russia’s pro-business camp, some observers have begun talking of a campaign against them by Kremlin hardliners, though officials deny there is any political motivation.

In April, Russian law enforcement officials searched the offices of oligarch Mikhail Prokhorov, whose interests include the Brooklyn Nets basketball team in the United States. The head of one of Prokhorov’s businesses is in jail, accused of unlawfully paying himself a bonus.

In June, investigators detained Nikita Belykh, a regional governor close to Russian business circles, on allegations of accepting a bribe.

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Two sources close to Vekselberg told Reuters the tycoon had met Putin at a business forum in Vladivostok at the weekend, and said that the tycoon was in good standing with the Kremlin. Putin’s spokesman, Dmitry Peskov, did not immediately respond to questions from Reuters.

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A Reuters reporter at the offices of Renova in central Moscow on Monday saw two armed men in ski masks with the insignia of the FSB security service on their uniforms. They were in the reception area of the offices.

Three people accompanied by an FSB officer entered the building, carrying what looked like a tool kit. Tools are sometimes used in searches to prise open safes or gain access to computer hard drives.

The Investigative Committee, the state body that investigates crimes, said the two people detained as suspects were Evgeny Olkhovik, Renova chief managing director, and Boris Vainzikher, chief executive of Renova subsidiary T Plus.

The subsidiary, previously known as KES, is the focus of the bribery investigation.

In a statement, the investigators also they were seeking to question Mikhail Slobodin, chief executive of Russia’s third biggest mobile operator Vimpelcom, a unit of New-York listed Vimpelcom Ltd. He is a former boss of KES.

Vimplecom said on Monday that it had accepted Slobodin’s resignation with immediate effect.

Kjell Johnsen, head of major markets, will lead the company’s business in Russia, Vimplecom said in a statement.

Vekselberg himself was not present at the time law enforcement were searching the Renova offices, as he had an appointment just outside Moscow, a source close to him said. He was informed of the searches, the source said.

The Investigative Committee said an inquiry had established that between 2007 and 2014 KES officials and affiliated organizations bribed senior officials of the northwestern Komi region to set advantageous heating and electricity tariffs.

It said the bribes totaled more than 800 million rubles ($12.34 million).

Renova group owns a 5.7 percent stake in aluminum giant Rusal, while Renova subsidiary Sual Partners, in which Vekselberg is a major shareholder, also holds Rusal shares. A Rusal representative said there were no searches at its office.