An undersea power cable running between Finland and Estonia was disconnected on Christmas Day. Finland is sure that Russia is to blame. On Thursday, Finnish authorities boarded an oil tanker that was part of Russia’s “shadow fleet” used to evade sanctions and happened to be spotted passing the EstLink 2 underwater cable as it stopped working, according to the Financial Times.

Per The Guardianthe incident happened at 12:26 pm local time on Wednesday, and Arto Pahkin, the head of operations in Finland for the electricity grid, said immediately that sabotage cannot be ruled out. Finnish authorities also confirmed that the damage was caused by at at least three other cablespossibly part of the same incident.

This led to the seizure of the Eagle S, an oil tanker registered in the Cook Islands but believed to belong to Russia. The ship’s tracking data suggested it was carrying oil from Russia to Egypt, but apparently made room to cause chaos along the route. Authorities believe the ship’s anchor, which was not visible to the ship, was used to cut the cables.

The Eagle S is believed by authorities to be part of Russia shadow fleetwhich the country has used since the start of its war with Ukraine to evade Western sanctions. The fleet consisted of old, rickety ships that Russia hid its ownership of using a variety of techniques including labyrinthine management structures, frequent transfers of cargo between ships, falsified information , identification system blackouts, and other schemes. It is assumed that the country is moving 600 ships as part of his off-the-books oil business. Since many of these ships carry oil and are poorly maintained, they often violate modern safety standards and ignore regulations, resulting in more damage including oil spills.

The Christmas Day cable cut was just the latest in a series of incidents where undersea cables connecting NATO countries have been cut. Last month, two fiber-optic cables in the Baltic Sea—one running between Finland and Germany, the other connecting Lithuania and Sweden—CUT OFF. Those, too, are believed to be the work of the Russian shadow fleet, which makes a habit of hanging around critical infrastructure in the Baltic and North Seas, according to the New York Times.

While the attacks on these cables have not yet resulted in any tangible disruption, they have led to an increase in concern about how this undersea infrastructure could be targeted in future conflicts. Reports suggest that China has used similar tactics in the past, using boat anchors to damage submarine cables. It’s quite a bit quick attack to makebecause most cables are about as thick as a garden hose and sit exposed on the ocean floor. Countries are exploring additional safeguards for these key pieces of connectivity to prevent future attacks.