This is an excellent, detailed, long twitter thread in 4 parts. I've linked the thread-reader version, or you can start from the first tweet, HERE .
QRD: In 1994 US submarine on exercise in the Baltic accidentally (?) sank a passenger ferry killing almost 1000 people. The Swedish government, and other Baltic nations who were set to join NATO, covered it up and blamed a design failure for the sinking. At the time, NATO was expanding eastwards. Since the fall of the USSR however, and with it the loss of the enemy NATO was established to defend against, this expansion seemed unnecessary to the public. The incompetence of the (apparent) accident apparently would have been a PR nightmare for NATO's pointless existence and expansion, at least under it's guise as a "defensive partnership". The cover-up was seemingly so crucial that actual survivors, who may have had witness testimony that would contradict the cover-up, were disappeared and/or apparently thrown back into the ocean (hours after being rescued and even cared for in hospital) to drown.
Crazy stuff.
Add to that the fact that the government quickly arrests anyone getting near the wreck, referring to it as a "grave site". They wanted to cover the entire ship in cement before even searching for bodies to recover. Which is all very odd from what's normal during an incident like this.
Every Swede knew already back then that this was no accident, the question how was revealed only a year ago when a diver managed to get close enough to get a proper picture, revealing a massive hole on the side which proved the submarine torpedo theory many had been speculating about.
The dead bodies are still there, all the personal belongings too, everything left untouched. Including whatever was on the ship which the government didn't want to arrive at it's destination, and ironically hasn't been able to recover to this day.
The NATO theory makes the most sense, normally we'd get to know the truth from official sources once all the lose ends has been tied up and there's nobody alive to punish for it. It's likely they want Sweden and Finland to join NATO first, then make the NATO membership protected by the constitution so that neither country can leave the union. First then, in at least 10 years it may be risk free for them to reveal the truth.
Those responsible are likely all dead by now anyway.