US response to horrific terrorist attack?

What would the US response be to a nuclear terrorist attack on a major city? This might seem like an easy question in some ways but what if the origin of the attackers can not be found, what if somehow they remain totally anonymous? How can a country respond to something like that? who would be bombed? where can u concentrate the revenge effort?

This is (at present) a hypothetical situation of course but i believe a situation of this type is entirely possible considering the number of terrorist organisations and even individuals who hate the States with fevourish conviction. Granted it would be very hard to get a nuke into the US but it is possible with the right equipment and planning, if this is possible then it follows that people can be smuggled in too without going through imigration control. A terrorist organisation knows that if their identity were to be found it would translate into a virtual middle east take over(assuming they are of some islamic organisation) and so it would be in their interests to cover their own tracks.
The people would demand action but the government would not know where to direct it, would the US gov ''find'' those responsible even if they couldnt? (i assume u know what i am getting at). Or do u think it is possible that nothing would be done if nobody could be found to be accountable for the attack?

[edit on 4-3-2005 by ufo3]