I think most of the countries investigating this has a main theory that a Ukraine backed group carried out the attack on the Nordstream pipelines? That said, I have a hard time believing anything coming from both US/EU and Russian related sources on this topic. Unless there are any hard evidence, we're only going to be able to speculate who did it.

https://www.theguardian.com/business/2023/sep/26/nord-stream-pipeline-blasts-key-details-revealed-by-scientists