I highly doubt they will try a "Dirty bomb", its to messy to assemble and transport. I think they will use a real atomic bomb. They can obtain them from Pakistan, after all Pakistan said their atomic bombs are ISLAMIC nuclear bombs to defend the muslim ummah. Pakistan has miniaturised their nuclear weapons and all they have to do is pass to Al Qaeda. This of course is relatuvely simple because the ISI and Al Qaeda are essentially one and the same.
The world should make it clear that if such a plot takes place then Islamabad, Karachi, Tehran, Medina and Mecca will become craters in the ground.
US State Department should stop restraining India and Israel.