Why do you think that would be hard? The military is not a democracy. It kills people.
I don’t think everyone on a ship with dozens to hundreds of crew would keep quiet, for twenty years.
A little boat with six? Sure.
Terrorists? Why would they? that’s the problem with it being a terrorist attack. Unless the purpose was to win concessions in a secret negotiation between them and the gov’t.
Its been 20 years. Impossible to believe everyone on a ship would keep quiet for that long. Most are probably long out of the military and pretty much untouchable.