Egyptian aviation authority and black box
Then it seems clear that even if the French will get involved into this investigation, they would not be able to effectively dispute the Egyptian intepretation of facts, especially as the investigators operate on Egyptian territory, even if the "French" investigation would prove that this incident was caused by a pilot error or terrorism.
This "cultural" difference in handling these delicate events it is something we should all consider in order to achieve an impartial judgement and fair investigation.
I doubt we will never know the truth, unless the French and Boeing will get full access to all the evidence.