Speculations
It is possible that the United States is being blackmailed by some unknown individual, group or nation, who/which I will refer to here simply as Agent X.
scenario:
Agent X contacts the authorities and makes some demand. Unless you do so-and-so, I will destroy one of your planes within 48 hours.
This is all speculation, of course, but imo, neither would the United States submit to the demands of the blackmailers, nor would it be likely (in such a case) to reveal the true cause of the disaster.
As I've stated before elsewhere, I think it is possible that back-channel communications of one kind or another may take place from time to time between "Agent X" and our authorities.
sp