the people involved can keep a secret, after all that would be their goal...(yet the evidence still exists).
Thats why the public knew nothing about the proposed U.S. government sponsored terrorism of Operation Northwoods until almost 40 years later.
http://www.whatreallyhappened.com/northwoods.html
http://www.gwu.edu/~nsarchiv/news/20010430/
http://en.wikipedia.org/wiki/Operation_Northwoods
Thats also why the problem-reaction-solution paradigm has been used so succesfully all throughout history...
- and thats why I offer it as a hypothesis