Most experts say that the risk of a nuclear attack is low but not unthinkable, because of the spread of nuclear materials around the world.

Scientists can analyze tiny bits of radioactive fallout that is invisible to the naked eye. It can reveal distinctive strains that describe the type of bomb, where it was manufactured and the country it came from.

A nuclear blast leaves clues that can help track down the specific terrorist groups.

[If a nuclear bomb blows up and kills everyone and everything in sight for miles, and can wipe out a country, what is left to find and who to find it?]