It doesn't have to be passengers who have done the hijacking. A concerted effort, such as this seems to be, could very easily involve flight attendents, pilots, grounds crew, or anyone else who would have legitimate access to the plane while on the ground.

Then again, when was the last time someone checked your shaving cream, perfume, cologne, or any other "normal" item to ensure that it really was what it was said to be? Would you have been willing to wait while the security check points really did squirt each can, bottle or whatever to ensure "complete" safety? It would not be difficult to get something onboard it one was determined to do so.