Finnish authorities have boarded a foreign oil tanker escorted into its waters after disruptions on a subsea power link and several data cables.

The police and border guard boarded the vessel, Eagle S, for investigations. The ship, sailing under the Cook Islands flag, was observed to be missing its anchor, Finnish authorities said at a news conference December 26. The anchor is believed to have caused the damage, they said.

On December 25, a 170-kilometer (105-mile) subsea electricity line connecting Finland and Estonia went offline. Since then, four underwater data cables have also been experiencing disruptions.It’s the third such incident in just over a year. In November, a high-speed fiber optic cable in the Baltic Sea connecting Finland and Germany was severed by what was likely an external impact and a nearby link between Lithuania and Sweden was also damaged, with authorities connecting Chinese bulk carrier Yi Peng 3 to the events. A year earlier, the anchor of NewNew Polar Bear, a Hong Kong-flagged ship, tore up at least two data cables and a gas pipeline, prompting NATO to step up patrols in the Baltic Sea.

“Damage to vital underwater infrastructure has become so frequent that it is hard to believe it was a coincidence or just bad seamanship,” Estonian Foreign Minister Margus Tsahkna said in a statement. “We must understand that damage to the undersea infrastructure has become systemic and must be treated as attacks on our essential infrastructure.”

Eagle S is part of the Russian shadow fleet transporting its oil, and was carrying a full load of unleaded gasoline, the police said. Finland’s National Bureau of Investigation is probing aggravated criminal mischief. The telecommunications cables are operated by Elisa Eesti As and CITIC Telecom CPC as well as Cinia Oy, including the Finland-Germany link that was also damaged and repaired in November.

It could take months to repair the 650-megawatt EstLink 2 power cable, operator Fingrid Oyj said.

Finnish and Estonian authorities are in close contact, and the North Atlantic Treaty Organization and the European Commission have also been informed, as have their neighboring countries, Finnish Prime Minister Petteri Orpo.

“This is another severe incident in a chain” of events, Orpo told reporters, calling for new measures to stop Russia’s shadow fleet. Western nations have struggled to find effective means of securing critical infrastructure in remote locations and on the seabed.

Shadow fleet vessels have been added on sanctions lists and earlier this month, 12 European countries agreed to introduce checks on proof of insurance from suspected shadow vessels as they pass through the Baltic Sea and the English Channel.

But Orpo had a warning to other ships that they could face the fate of Eagle S.

“The vessel is now under our control and that’s a strong message to others sailing the seas — we will step in,” Orpo said. “We cannot sit back and watch this happen. And we won’t.”