I'm not sure what the legitimate national security concern would be for "we accidentally shot down an airliner 26 years ago".
How does that work when everything is classified?
Well, they're - vaguely - claiming to have found a smoking gun in FOIL requests.
It's also fairly likely some of the remaining classified docs include "we asked our mole at the Russian embassy to check into it discreetly" and "spy satellites checked to see if Russia was quietly mobilizing in advance" sort of scenarios likely to remain indefinitely secret.