US, Israel, UK, Russia, Germany, France, Australia, India, Pakistan, China.
These countries’ spy agencies are probably the only organizations capable of planning and executing such a flawless, precise, and perfect operation without detection or discovery. Now, the questions comes down to WHICH one and WHY?
“You have agents, disguised as normal citizens, on the plane with the authority and means to execute various plans to cover a variety of emergency situations, including learning of a WMD onboard.
The US has armed federal marshals on domestic flights, so it would be reasonable to assume they have equivalent international agents on international flights, especially from certain, suspect countries.”
No, that’s not reasonable to assume. How many flights are we talking about, where you would need multiple agents on every flight, to guard against a frankly not very likely scenario? We don’t even manage to get one Air Marshal on every domestic flight, so where’s the budget to pay for this?
Also, we haven’t even taken the minimal steps to ensure competent domestic security for our flights, so why would we assume that the same incompetent government could manage to protect international flights better than our own?
Sorry, meant to reply to the OP, not you.