In the absence of evidence and with only finger-pointing at this stage, I am inclined to believe that Russia is responsible since they have damaged their own pipelines several times in the past and lied about those instances, too. If Russia had any credibility before, they have essentially none now, and they do not deserve the benefit of the doubt.
As for their motives, there are several: to avoid paying the "force majeure" penalty, to give an unambiguous signal that no more gas is forthcoming in the immediate future, to threaten Europe's own energy infrastructure with a demonstration, to weaken the alliance by making some EU countries suspect the US, and for Putin to warn Russia's oligarchs that they can't just depose him and resume gas shipments - Russia is now all-in and can't go back to normal anytime soon even after a regime change.
As for Biden saying he would "stop" the pipeline earlier this year, he was talking about a different pipeline, Nord Stream 2 (which had not yet started operation at the time he made that remark), not Nord Stream 1. He was threatening to work with US allies to block it from opening in the first place.
The US has forbidden Ukraine from attacking Russian territory with US weapons, why would we take the escalatory step of attacking Russian infrastructure ourselves? We could have just let Ukraine do whatever they wanted with US weapons and denied responsibility, but the US was careful to avoid provoking Russia too much. It makes no sense that after all that tip-toeing around we would say "to hell with it" and launch a direct attack.