How much metal is in a razor blade to set off a metal detector? You could hide a razor blade in your pocket. For that matter, there are other things that could be used like knives that wouldn't be detected except by a really thorough search.
From the accounts that are coming out, these guys were armed with some *really* crude weapons, not requiring any inside help at all. Razor blades stuck in plastic handles? And the passengers just sat around while they started cutting throats?