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The attack, which happened as Europe attempted to wean itself off Russian energy sources following the Kremlin's full-scale invasion of Ukraine, contributed to tensions that followed the start of the war. The source of the sabotage has been .

Denmark, Sweden and Germany have been investigating , and the Danish Foreign Ministry tweeted a letter Tuesday from the three countries' U.N. ambassadors to the president of the Security Council with information on their activities so far.

Officials in March over media reports that a pro-Ukraine group was involved in the sabotage. German media reported then that five men and a woman used a yacht hired by a Ukrainian-owned company in Poland to carry out the attack, and that the vessel set off from the German port of Rostock.

German federal prosecutors declined to comment directly on that and other reports, but they confirmed that a boat was searched in January, and said there was suspicion that it could have been used to transport explosives to blow up the pipelines.

A section of this week's letter detailing Germany's findings said that the yacht's precise course had not been definitively established. The letter said “traces of subsea explosives were found in the samples taken from the boat during the investigation,� but it did not elaborate.

“At this point it is not possible to reliably establish the identity of the perpetrators and their motives, particularly regarding the question of whether the incident was steered by a state or state actor,� it said. “All information to clarify the matter will be pursued during the continuing investigations.�

The undersea explosions ruptured the Nord Stream 1 pipeline, which was Russia's main natural gas supply route to Germany until Russia at the end of August.

The blasts also damaged the Nord Stream 2 pipeline, which never entered service because Germany shortly before Russia invaded Ukraine in February 2022.

The pipelines were long and some of its allies, who warned that they posed a risk to Europe’s energy security by increasing dependence on Russian gas.

Russian President Vladimir Putin and Russian officials have accused the U.S. of staging the explosions, which they have described as a terror attack.

Ukraine has rejected suggestions that it might have ordered the attack. The countries investigating the explosions have not commented on who might have been responsible.

Since the blasts, NATO has boosted its presence in the Baltic and North Seas, using dozens of ships, aircraft and undersea equipment such as drones.