Steal a years supply of polonium 210 americium from a smoke detector factory. Grind it into a powder and bulk it out with something more pedestrian but nasty - say, powdered beryllium. Set it on fire and have the smoke get sucked into the ventilation system of the Empire State building. The beryllium smoke gets you a high body count, but the radioactive part will get you the headlines.
Fortunately most terrorists seem to either not know a lot about chemistry or not be very imaginative.
Edit to fix stupid mistake.
