While I think that the above theories are obviously incorrect (i.e. EgyptAir 990 was not downed by the US; and the FDR tapes did not reveal any attempt to turn back to NYC), I do feel that some kind of link between the incidents should be explored by the relevant security agencies.
Was the EgyptAir 990 pilot who put the plane in the drink (I forget his name) in any way linked with any of the highjackers? Had he had any meetings with them? Did their careers cross paths at any time?
It may be conceivable that he did have a link with them, and that his knowledge of what they were going to do so distorted his brain that he decided to take his own plane down first.....
After all, he was a 767 rated pilot, and his last repeated phrase did indicate some degree of religious devotion.
Just a thought...
If it was a suicide, at least the EgyptAir 990 pilot did not take out anyone on the ground with him. Just the poor souls in the plane. As we now know, it could have been much worse.