Finkelstein, your hypothetical situation has several flaws. Through no fault of your own since you don't seem familiar with nuclear weapons.
First, any bomb that ISIS might get would be in the kiloton range (1 kiloton = 1,000 toms of tnt). That would not be able to kill 10 million people; it would not even be able to destroy all of Manhattan with a population of 1.6 million. Hundreds of thousands maybe.
You also underestimate our ability to find out a lot through forensics as to the origin of said devices. They can be traced back to its country of origin because the radioactive residues of Uranium have their own particular radioisotope "fingerprint" depending on who produced it.
There are much easier ways of wreaking havoc, which I won't detail, that can kill thousands without any explosion. It would be a ridiculously small weapon whose raw material is easily available. It's a good thing that these terrorists have little imagination; they'd be much more effective if they did.