We know very little about the author of this letter other then he has credentials of unknown value. We don't know the real value of his credentials or the reason he resigned. To be giving the letter any sort of credit at this point is very premature.
Beside that, a little common decency would cause most people to wait a few days before going on their tirades of blames. There will be plenty of time for finger pointing and scoring of political points in the months to follow...a decent interim period of calm in honour of the dead doesn't hurt.
I mean WTF, We havent even gathered enough of the remains suitable for burial and these tinfoil hat wearing Bush Bashing imbeciles are out in full force.