It's possible that a pilot or flight attendant was involved and expedited the highjacking. I think service workers (e.g., the guys who deliver the in-flight food) can pretty easily smuggle weapons aboard. You can hijack with a knife as well as a gun, and, for example, I have a tanto (sort of like a Samurai sword only much shorter) which is made of very rigid plastic impregnated with glass fibers. Not much good for slicing but it can stab just fine. That sort of thing can go through X-ray machines without even being noticed.
There really isn't much we can do to prevent attacks like this, especially when the attackers don't even care about their own lives.
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