Ukraine

I thought the investigation was closed inconclusive more than half a year ago?

Last I've heard is that Germany put out an arrest warrant for an Ukrainian (not saying it was Ukraine) and Poland failed to arrest him for months, then said "whoops he just crossed the border". They also very much failed to hand over relevant evidence, or at least clues, e.g. surveillance tapes in an harbour.

But that doesn't even mean that it was Poland -- they have motive, but it's also not really in character, they wouldn't want to piss of Germany much less when Germany is in the process of getting rid of Russian gas. I still haven't ruled out Germany, an investigation going on and on doesn't point in the opposite direction of course they'd let the state attorney investigate to their heart's content while simultaneously working with the intelligence services of other countries to turn every lead inconclusive and elusive.

Ukraine, as far as I can tell, doesn't have the capability. US? I don't think so, either, their submarines are way too loud (because nuclear), people would've noticed. They're just not made for covert operations in the Baltic Sea. Manoeuvring two depth charges into place from a yacht seems to be grasping for straws.

Russians are also suspects. Yes it's their pipeline, yes it'd have been easier for them to explode the pipes from within, but they might have orchestrated a false flag out of spite.

Chances are we won't know until the war is over, or even a decade or two later. One thing is for sure: It was done by people who knew what they were doing.