INVESTIGATORS found traces of sub-sea explosives in samples taken from a yacht that has been one aspect of a probe into the sabotage of the Nord Stream gas pipelines under the Baltic Sea last year.

Reporting to the United Nations security council on Tuesday, European diplomats said the investigation hasn’t yet established who the perpetrators were and whether a state was involved.

Denmark, Sweden and Germany have been investigating the September 26 attack, and the Danish Foreign Ministry tweeted a letter on Tuesday from the three countries’ UN ambassadors to the president of the security council with information on their activities so far.

“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. 

Officials voiced caution 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 it set off from the German port of Rostock.

The award-winning reporter Seymour Hersh said in February that US navy divers planted explosives that destroyed the three Russian gas pipelines.

The White House dismissed the report as “utterly false and complete fiction.”

The New York Times, The Washington Post and German media published stories in March citing US officials as saying there was evidence Ukrainians may have been responsible. 

The Ukrainian government has denied involvement.

Russian President Vladimir Putin has dismissed as “sheer nonsense” allegations that Ukrainians could have been behind the blasts and pointed the finger at the US.

The pipelines were long a target of criticism by the US, which warned that they posed a risk to Europe’s energy security by increasing dependence on Russian gas — while seeking new markets for its own fracked shale gas.