The US is the most likely suspect (they have the means, motive, and opportunity), but AFAIK no positive evidence has been published. One of the countries investigating the sabotage stopped releasing info to the other investigating countries, which might mean they have some sort of evidence. If a NATO signatory were to release evidence that the US had sabotaged the pipeline, then the US would try to expell that signatory from NATO and declare it a Russian puppet state. So, I don't expect to see any positive evidence until long after the US-Russia rivalry is over.