"2. unrelated accidents (has this ever happened before) "
ok I am now reading that bad fuel can do this....it would certainly account for #2. Are there any other mechanical issues what could plausibly cause 2 planes to go down in order?
A terrorist mechanic...
But, were it bad enough fuel to take down two, we'd probably be wondering about the number three, four, etc.
bad fuel would leave time for a mayday call, no?