The kremlin wants to deliberately destabilize Europe and make it as difficult as possible with a big energy crisis, so support and aid to Ukraine falls. And its going bad for putin and his war now so he is also desperate, just look at the crazy mobilization of 300.000 russian to the war.
The russians already know that these pipelines are dead and the export trough them will end and sabotaging them is very much in the russians interest because of the consequences and as we can see now increased energy costs in EU which benefits the russian agenda.
Some say its US because Biden talk. But both pipes was already closed. Nordstream 2 was never used and nordstream 1 was shut down by the russians. The US hade no reason because no gas was flowing.
IMHO its no doubt its the russians, they are the only one that gain from this, in their energy war against EU. They also have submarines in the baltic sea. And its shallow waters between Denmark and Sweden (just 11-18m) that big subs from other countries can not easy pass without getting noticed and a lot of traffic that make it risky.