I'm sure there has been some type of information acquired by our guys. Whether that is an increase in communications, recorded comm, more "suspects" entering the country than usual, etc. There has to be some indicator that would cause them to give warning. Granted, it may all be a ploy by terrorists. That is the game these guys play. One big successful attack such as 911, and they can milk it for years while our complacency increases to the point of ignorance, and they have to attack again to get our attention.
"But, but, but.......this is the mostest serious since 911. I mean it!! I'm not kidding this time........."