A former owner of the debt-laden Antipinsky refinery, Russia’s largest independent oil-processing plant, was arrested in Moscow, his company said.

New Stream group said its head, Dmitry Mazurov, was arrested by the Investigative Committee on Saturday at Moscow’s Sheremetyevo airport when he arrived back in Russia.

It said Mazurov was a suspect in a criminal case, without elaborating. New Stream formerly owned the refinery.

Authorities were not immediately available for comment.

Last month, SOCAR Energoresurs, a joint venture between Russia’s largest lender, Sberbank, and a group of investors, acquired an 80% stake in the Antipinsky refinery.. Sberbank is the plant’s main creditor.

The plant, which has an annual capacity of 9 million tonnes, filed for bankruptcy in May, weeks after a London court ordered its assets to be frozen in response to a lawsuit from a trading house.

Court data showed there were 346.5 billion roubles ($5.5 billion) of claims against the refinery.