Finnish authorities have secured an oil tanker sailing under a Cook Islands flag, which it believes to be part of a Russian ‘shadow fleet’ transporting embargoed hydrocarbons. Helsinki suspects the ship, the Eagle S, of being involved in the damaging on December 25 of the EstLink 2 underwater electricity cable between Finland and Estonia.

Sweden announced on Friday it would be sending a submarine rescue vessel to assist in the investigation.

“With their unique expertise, the Swedish armed forces are contributing to helping Finland shed light on what happened,” Sweden’s prime minister, Ulf Kristersson, said in a statement.” “Since the discovery of these cable breaks in the Baltic Sea, we have said that Sweden stands ready to help our neighbors.”

“The security situation is serious and we want to strengthen security and cooperation with neighbors and allies,” Kristersson added.

The Eagle S has been escorted to a port near Helsinki, where investigators are inspecting it and questioning its crew of about 20, the Kyiv Post reported.

NATO has announced plans to increase its presence in the Baltic following a spate of similar incidents, attributed to “hybrid war” tactics of Moscow.