The Finnish police on Thursday boarded an oil tanker that the authorities said they suspected was involved in damaging vital undersea cables and suggested was part of a shadow fleet used by Russia to avoid Western sanctions.

The Estlink 2 submarine cable, which carries electricity between Finland and Estonia, was cut on Wednesday, Finland’s police said in a statement, the latest in a slew of disruptions to undersea infrastructure that are being investigated as acts of sabotage. The Finnish authorities said Thursday that four other cables carrying data also had been damaged.

A number of other vital undersea cables have been cut in recent months, raising fears that Russia might be waging a shadow campaign against NATO nations that have supported Ukraine in the face of Moscow’s full-scale invasion.

Last month, two fiber-optic cables were cut in the Baltic Sea in what Germany’s defense minister described as an act of sabotage. One cable connected Finland and Germany; the other ran between Lithuania and Sweden — all members of the NATO alliance.

Russian ships have been reported in the Baltic and North Seas near areas where critical infrastructure lies beneath the waters, and dozens of Russian tankers have begun sailing under different flags to evade sanctions.

The police in Finland called the latest cable cuts “aggravated vandalism.” In a statement on Thursday, the police said the authorities had seized the Eagle S tanker, which is registered in the Cook Islands in the South Pacific. The ship was sailing from St. Petersburg, Russia to Port Said, Egypt, when it entered Finnish waters.