Russia has a fleet of suspected spy ships operating in Nordic waters as part of a program for the potential sabotage of underwater cables and wind farms in the region, according to a joint investigation by the public broadcasters of Sweden, Denmark, Norway and Finland.

The broadcasters used data analysis, intercepted radio communications and intelligence sources to show how around 50 boats had been gathering intelligence for the past 10 years, using underwater surveillance equipment to map key sites for potential sabotage, Norway’s NRK and Sweden’s SVT reported.

The Russian vessels have been sailing past military training areas, important oil and gas fields, small airports, deep-water quays, and strategically important hubs for the Norwegian Armed Forces, according to NRK. The investigation also said Russian ships appear suddenly following NATO exercises. Norway and Denmark are founding members of NATO, while Finland joined earlier this month – to Moscow’s displeasure – and Sweden is seeking to follow suit.

One ship at the center of the investigation, the Admiral Vladimirsky, is officially used for underwater research expeditions, but is, according to the report, a Russian spy ship. The broadcasters tracked the boat’s movements and found it had traveled for a month through Nordic waters with its transmitter turned off to remain hidden, sailing close to wind farms, as well as through naval training areas of the Swedish Armed Forces, SVT reported.

When a TV crew from Danish public broadcaster DR approached the ship on the sea between Sweden and Denmark, a masked man emerged on deck who appeared to be wearing body armor and armed with an assault rifle, with the incident caught on camera.

The investigation also said that the Russian fishing vessel Taurus, which delivered fish to Norway between 2015 and 2022, has appeared in unusual places, heading straight towards military firing ranges and close to military bases where all naval traffic is prohibited. In one case, it appears Taurus had stopped fishing and gone directly to the city of Ålesund, passing by a major NATO exercise in the region of Trøndelag, NRK reported.

“We see it very clearly now. Russia says cooperation with the West is over, now it is confrontation,” the head of the Norwegian Intelligence Service, Nils Andreas Stensønes, told NRK in response to the report.

The investigation comes after Dutch intelligence officials warned Russia had tried to gain intelligence to prepare for the potential sabotage of critical infrastructure in their patch of the North sea.

“We saw a couple of months ago that Russian ships, a Russian ship, wanted to enter the area where Dutch windmill parks in the North Sea are located with the intention to see how the command and control structure of these windmill farms, how it is operated,” Jan Swillens, head of the Dutch Military Intelligence and Security Service (MIVD), told reporters. “So that was a threat … It was the first time that we saw a Russian vessel enter the area of the windmill farms to do their reconnaissance.”

Royal Norwegian Naval Academy researcher Ståle Ulriksen, who has been assisting with the investigation, told NRK he believes there could be more than 50 Russian spy ships in Nordic waters. “We are talking about a very large system. Quite a large fleet. Altogether, we are talking several hundred,” he was quoted as saying.

The first of several reports will be broadcast Wednesday on DR in Denmark, NRK in Norway, SVT in Sweden and Yle in Finland.

Diplomatic tensions between Moscow and Helsinki have been heightened following Finland’s accession to the alliance. The country scrapped its longstanding position of neutrality to become a NATO member as domestic support for joining the alliance surged following Russia’s invasion of Ukraine.

The accession marked a major shift in the security landscape in northeastern Europe, adding some 1,300 kilometers (830 miles) to the alliance’s frontier with Russia.

At the time, Moscow warned that further NATO expansion would not bring more stability to Europe, and said it would scale up forces near Finland if the alliance sent any troops or equipment to the new member country.