Crew on board an oil tanker accused of sabotaging undersea power and communications cables in the Baltic Sea were poised to cut other cables and pipelines when Finnish authorities boarded the vessel last month,

Swedish authorities said late on Sunday they are detaining a vessel suspected of “gross sabotage” following the damage of an undersea communication cable in the Baltic. “Concrete investigation measures” are being carried out on board the ship, the Swedish prosecutor office said in a statement.

A Bulgarian shipping company on Monday denied that one of its ships had intentionally damaged an underwater fiber optic cable connecting Latvia and the Swedish island of Gotland

NATO is ratcheting up its guard against suspected attempts to sabotage underwater energy and data cables and pipelines that crisscross the Baltic Sea.

The Nordic country has opened an investigation into the damage, just weeks after NATO stepped up its military presence in the area following a series of similar incidents.