I think it's overwhelmingly obvious that the US, or US-aligned forces, were responsible. It's not clear to me who else even has a motive to do this. US has capable forces in the region at the time, a clear motive (remove the temptation for Germany to use the pipeline). The other possibilities are Russia, they could just turn off the pipeline rather than blow it up, Germany, they could just turn off their end of the pipeline without blowing it up, or... Ukraine, Poland, etc I count as "US-aligned" forces.
Then, when you add in that Biden explicitly threatened unilaterally stopping the pipeline, and that threat was subsequently reiterated by the head of the State Department... I have to wonder what evidence would convince people that the US was probably behind the action.
> It's not clear to me who else even has a motive to do this.
Putin. Blow up the pipeline so no-one can offer cheap gas in exchange for being recognized as the next legitimate leader of Russia behind his back. It's one of the simplest explanations and falls into the overall pattern. False-flag terrorism[1] is how Putin got into power in the first place.
Cheap gas or not, the west would welcome any new Russian leader with open arms if they withdrew from Ukraine. That explanation does not seem very compelling at all.
The west is not the only party that would have to welcome a new russian leader - the russian oligarchs, as well as people interested in making money selling to the west, needs to be on board.
Putin can remove the temptation to overthrow him, and restoring normal relations with the EU (gas pipes included), by blowing it up - something that is currently not being used and serves to short to medium term value for putin. However, blowing it up, but hiding who did it, can sow mistrust in the NATO alliance. He might be betting that the germans would think the americans did it (or at least be suspicious). And if he was found out, russia can claim that they're within their rights to blow up a pipeline that belongs to them (aka, no real bad consequences).
What? You could repair the damage to the pipeline. If someone could replace Putin by promising to sell gas to the Germans, the fact that the pipeline is currently damaged would not be an obstacle.
It's also not the case that Putin needs false flag terrorism. For one, he is already in power and has already started the war. Two, if he needed an example of US or Ukrainian forces blowing something up to galvanize his position - he has plenty of real world examples of Russian things getting destroyed - he doesn't need to manufacture a new one.
It was damaged beyond repair, certainly beyond a quick repair. If the plan with gas extortion was to break the will of Europe over this winter, then no alternative to Putin could offer any relief after it was blown up, decreasing the value of backchannel negotiations.
Then, when you add in that Biden explicitly threatened unilaterally stopping the pipeline, and that threat was subsequently reiterated by the head of the State Department... I have to wonder what evidence would convince people that the US was probably behind the action.