"They'll give out the descriptions when they're good and ready, along with other info"
...meanwhile the perps have boarded a plane and are on their way to who knows where. Why not give a description while its early enough in the game for some citizen to recognize them? (..unless they really have no good descriptions at this point--quite possibly the case!)
I think the descriptions they're likely to have gotten will be something like "Young muslim-looking guy" which covers a couple of hundred thousand of the grumpy commuters trying to get home right now. So far we've had zero casualties ...
They might not want the terrorists to know whether they have "little fish" or "big fish". While they all are working together, they might not know each other's positions.