STORY: The Swedish Coast Guard on Thursday said it discovered a fourth unexplained gas leak in the undersea Nord Stream pipelines linking Russia to Europe.
And as gas rose to the surface, allegations of sabotage flew between Brussels, Moscow and Washington.
The European Union is investigating the cause of the leaks in the Nord Stream 1 and 2 pipelines under the Baltic Sea and has said it suspects sabotage, to which it promised a "robust and united response." But it has not said who is to blame.
From Brussels, NATO Secretary-General Jens Stoltenberg warned "any deliberate attack against Allies’ critical infrastructure would be met with a united and determined response."
And an EU official drew a direct link between the Nord Stream leaks and the Russian invasion of Ukraine, saying about the possible sabotage of pipelines, "this fundamentally changes the nature of the conflict as we have seen it so far."
It's unclear why any nation, either supplying energy or consuming it, might seek to sabotage key energy infrastructure.
"I would be very doubtful that it's going to be any European state, why would any European state do this? And, again, I doubt it can be the U.S. also. So this is why I think the most logical - even if it's not the logical element - points to Russia."
Thierry Bros is a European gas expert at Sciences Po university in Paris. He told Reuters it's far from certain that Russia might seek to sabotage its own multi-billion-dollar energy infrastructure.
But he said that of all the possible perpetrators, Russian President Vladimir Putin may be the most likely, as part of a wider campaign by the Kremlin to highlight the fragility of Europe’s energy supply.
"Vladimir Putin is in fact making two wars: one, military one against Ukraine, that he might lose, but one he started quite a long time ago against the EU, against EU values. He is using loads of tools to do this, disinformation and weaponization of gas."
Russia and European partners spent billions building the pipelines, which are operated by Russian energy firm Gazprom.
Moscow - conducting its own investigation - has said it suspects what it called "state-sponsored terrorism," and a Russian foreign ministry spokesperson on Thursday said Washington had the most to gain from the damaged pipelines.
The Kremlin has also said allegations of Russian responsibility were "stupid."
Moscow had halted deliveries via Nord Stream 1, saying Western sanctions had hampered operations. Nord Stream 2 had not started commercial operations. Although neither was in use at the time of the leaks, both still contained large volumes of gas.