Are terrorists now actively recruiting and training "clean" candidates for these missions? You can be pretty sure they will cover as many bases as possible. The fellow that emerged from the head with the cell phone and ran his finger across his throat, mouthing the word "No" to the others, did he realize that they had been made or was he getting direction from someone on the ground at the airport? Maybe they had their own intelligence on the ground that noticed the greeting party waiting for them at the airport?
So many scenarios and questions...
My guess is that the flight crew were told by the Fed. Marshalls to leave them alone, so they could observe their behavior. They (the flight crew) knew the Feds were on board. I'm guessing they were just as concerned as the passengers.