That may reduce the possibilities...
Depth is moderate, in the neighborhood of 250-320 feet. A surface boat could launch an ROV carrying a shaped-charge mine for delayed detonation.
I heard a semi-credible report that we had an amphibious task force in that area a few days ago. When you think about it from several different angles the end point is the same.
The EU, in particular Germany (the cornerstone of the EU) is not lately a good friend to us nor are we to it. We have no love lost for Russia either. The destruction of NS-1 and NS-2 screws them all while increasing demand for US LNG.
The US had motive and certainly had opportunity.
If it’s below easy scuba depth, it’s almost certainly a nation-state. ROVs are cheaper than they used to be, but placing mines at depth requires testing.
So, cui bono?
Or, perhaps it’s better to ask not who benefits but who has a leadership that thinks they’d benefit, or has a military or intelligence service that is loosely-enough controlled to think they’d benefit?
One thing, though, the Baltic is a fairly constrained body of water. Nobody is sneaking submarines into it, and surface ships are tracked. There aren’t many nations of the world for whom operating even a civilian ship in the area wouldn’t be red flag.
It could have been the US, but my money would be on Poland.