FRANKFURT, Germany (AP) — The Swedish Coast Guard said Friday it has boarded a tanker suspected of causing a 12-kilometer (8-mile) oil spill on the Baltic Sea, adding that the vessel is under European Union sanctions aimed at the “shadow fleet” transporting Russian oil.

The spill was detected early Thursday east of Gotland island, and investigators identified the Flora 1 as the suspected source. The Coast Guard boarded the vessel early Friday and took it and its 24-member crew to anchorage near Ysted in southern Sweden.

“We act when we detect emissions. This is a result of our enhanced maritime surveillance that we are conducting as a result of the deteriorating security situation in the Baltic Sea region,” Daniel Stenling, deputy chief of operations, said in a statement. There were no immediate details on what caused the spill.

It was unclear under what country's flag the vessel was traveling and its destination was unclear, the Coast Guard said. It departed the Russian port of Primorsk, where a major Russian oil export terminal is located, on Tuesday, according to the MarineTraffic maritime data company.

The EU had sanctioned the vessel, meaning transactions involving it are forbidden, for carrying Russian oil while “practicing irregular and high-risk shipping practices." Unsafe practices can include turning off its location system to hide where a vessel is going.

The Ukrainian government says the vessel is owned by a Hong Kong company and has been affiliated with an Indian company that is “one of the leading operators of the so-called ‘shadow’ fleet involved in the transportation of Russian crude oil.”

A Ukrainian government website on sanctions said the Flora 1 carried the flag of Sierra Leone but had falsely used Benin as its flag country, and in the past had been detected carrying out a ship-to-ship oil transfer near Greece — one way of obscuring where oil comes from — and turning off its vessel location system.