Finnish Prime Minister Petteri Orpo raised concerns around Russia's shadow fleet of oil tankers after a vessel was seized on Thursday as part of an investigation into a ruptured undersea cable.Jussi Nukari/Lehtikuva/AFP via Getty ImagesAn undersea cable in the Baltic Sea that provides power to Estonia was cut on Wednesday.Finnish authorities say they've seized an oil tanker they suspect caused the outage.Finland's president raised concerns of Russia's "shadow fleet" on social media.Finnish authorities said they've seized an oil tanker on Thursday as part of its probe into the cutting of an undersea cable in the Baltic Sea which provides electricity to Estonia.Finnish customs authorities and the European Union's executive commission said the tanker may be part of Russia's "shadow fleet" of oil tankers, The Associated Press reported.