I do believe one thing: with US AWACS and other assets, and in that part of the world, I believe that the US government (and perhaps other governments, like Germany) know who did it. There a many reasons why they would not share information, such as an ally could have done it. Or, of course, Russia may have done it.
I am glad this story is posted on HN because I like getting many different opinions in one place.
In either the case of America attacking a pipeline jointly owned by the Russians or the Russians attacking a pipeline jointly owned by a NATO country, you could call this an act of war. But in neither case will either side use it as their casus belli to start WW3, because nobody wants that. Nobody is waiting for a 'legitimate' excuse to start such a war. It is much safer for everybody to pretend they haven't noticed such attacks, or to pretend that the culprit is uncertain or maybe that it was done by accident. Pretending to be ignorant of the provocation is preferable to WW3, for both sides of the conflict.