[supercat]

I believe you're correct; the maintenance/ground personnel were part of the plan and planted box-cutters in some planes. I think one was also found in a trash bin on board one flight.

If they were going to plant something, why would they plant something that could be carried on legally? Why not something more useful like an AK47 with a half-dozen spare magazines or an 18" short sword?

[Cachelot]

If they were going to plant something, why would they plant something that could be carried on legally

Someone might ask questions about knives, or demand that they be placed in the "main" luggage. I have experienced that myself, in Europe. Although I have always been allowed to have my swiss army knife, but I suspect that's a bygone.

And a box cutter found accidentally, by some random stranger, wouldn't raise near the interest that an AK would.