Anchor drag marks have been found on the seabed by Finnish police in an ongoing investigation into damaged power cables in the Baltic Sea.
Finland's National Bureau of Investigation (NBI) said the anchor marks were likely from a Russia-linked vessel that had already been seized on suspicion of breaking the undersea cables.
NBI's chief investigator Sami Paila said the anchor drag trail extended for "dozens of kilometres … if not almost 100 kilometres".
The owner of the oil tanker is seeking the release of the ship, a lawyer representing the company said.
Finnish police and coastguard officials boarded the Cook Islands-registered Eagle S and brought it to a location near a Finnish port where crew members were being questioned.
Baltic Sea nations have been on high alert after a string of outages of power cables, telecom links and gas pipelines since Russia invaded Ukraine in 2022.
Investigators said they believed the Eagle S broke the Estlink 2 undersea power cable linking Finland and Estonia.
It severed or damaged four fibre optic lines by dragging its anchor across the seabed for dozens of kilometres, police said.
The offence to rupture the cables is "aggravated interference of communications," Finland police said in a statement.
Police added that data of the vessel's passage was seized and "is being analysed".
Finland's president last week said he believed further damage would have occurred on the seabed had the ship not been stopped.
Finland's customs service has said it believes the Eagle S is part of a shadow fleet of aging tankers being used to evade sanctions on the sale of Russian oil.
It has formally impounded its cargo, although it is still on board the ship.
Owner wants boat released
United Arab Emirates-based Caravella LLC FZ is the owner of the seized vessel.
It filed a request with the Helsinki District Court to cancel the seizure of the ship.
Finnish lawyer Herman Ljungberg, who filed the documents on behalf of the company, said authorities had not provided any explanation of the legal basis for taking the vessel into custody and boarding it.
"The Finns have hijacked a vessel," Mr Ljungberg told Reuters.
He said the crew had been interrogated without any legal assistance, and they had been deprived of sleep.
A police spokesperson said the seizure of the vessel had taken place according to Finnish law and that crew members had been informed of their rights, including that of legal assistance.
They had not been deprived of sleep, the spokesperson added.
The NBI confirmed it is examining the alleged incident as a case of "grave sabotage", and that it may take several months to complete.
Growing concerns over shadow fleet
The Eagle S is flagged in the Cook Islands but was described by Finnish customs officials and the European Union (EU) executive commission as part of Russia's shadow fleet of fuel tankers.
The shadow fleet consists of aging vessels with obscure ownership acquired to evade Western sanctions amid the war in Ukraine.
Russia's use of the vessels has raised environmental concerns about accidents given their age and uncertain insurance coverage.
In the wake of the cable rupture, NATO Secretary-General Mark Rutte said last week that the military alliance, which Finland joined last year, will step up patrols in the Baltic Sea region.
EU foreign policy chief Kaja Kallas said that "sabotage in Europe has increased" since Russia's full-scale invasion of Ukraine in February 2022.
Ms Kallas said recent "sabotage attempts in the Baltic Sea are not isolated incidents" but "part of a pattern of deliberate and coordinated actions to damage our digital and energy infrastructure".
She vowed that the EU would "take stronger measures to counter the risks posed" by vessels of Russia's shadow fleet.
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