STOCKHOLM (AP) — A Swedish court on Sunday ordered the detention of the Russian captain of a ship that was suspected to be sailing under a false flag in the Baltic Sea and was boarded by authorities last week.

The commander of the Sea Owl 1, whose name hasn’t been released, was arrested on Friday — the day after the Swedish coast guard boarded the vessel off Trelleborg, on Sweden’s southern coast.

Prosecutors suspect him of using a false document. They said Sunday that the district court in Ystad ordered him held in custody in line with their request, Swedish news agency TT reported.

The tanker was sailing under the flag of the Comoros, an island nation off East Africa. But the coast guard has said that it suspects it isn’t in the shipping registry there and therefore there is no flag state to vouch for safety on board.

The tanker is also on the EU sanctions list and had been traveling from Brazil to Russia, according to the coast guard. It was previously used to transport oil between those two countries though it did not appear to have cargo on Thursday.

It was the second vessel sailing in Swedish territorial waters to come under coast guard investigation in a week under suspicion of using a false flag. The cargo ship “Caffa,” sailing with a majority Russian crew, is accused of transporting stolen grain while on Ukraine’s sanctions list. Its captain also has been detained on suspicion of using a false document.

Sweden last year said it would step up insurance checks on foreign ships in a move aimed at tightening controls on Russia’s so-called “shadow fleet ” of aging ships, which are used to transport oil and gas or to carry stolen Ukrainian grain.