These guys were smart enough to take flying lessons for airliners in preparation for their act. I would think that part of their training to become commercial pilots would be a course in airline security practices. With the instruction behind them I think the boxcutter loophole was one of many they had to choose from. Even a pencil can be quite an effective weapon, imagine what you could do with a knitting needle (still on the permitted list). With a guitar string (permitted) and any of a number of things to use as handles a garrotte could be fashioned easily. Even with the metal detectors they use today you could this knife or this one thru. Then again, if you just rip the toilet seat off and break it in half you'll have a couple of dandy clubs to wield. People complain now that the measures that have been taken post 9/11 have gone to far and as you can see, there are still plenty of loopholes wide open
As for the intelligence services, who knows. Those people are paid to do a job but they're also trained to cover their tracks. I don't think we'll ever know what part our intelligence agencies played in this event.

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