You would guess wrong.
It is not a matter of IF, it is a matter of WHEN.
I would be very, very surprised if Bin Laden or some other group did not already have a weapon or the materials to make it.
I would not be surprised if one or more cells in place in the U.S. had one.
It does not need to be in a "suitcase". It could be rolled into town in a Ryder van. It could float into a port onboard a freighter.
And, BTW, I am a rocket scientist!
--Boris