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Simpler still is the so-called dirty bomb. Detonated in a crowded city, a dirty bomb would pack an explosive punch no greater than ordinary ordnance, but the radioactive debris it would scatter could sicken and kill unknown numbers of people and contaminate an unknown stretch of real estate. Because the bomb would require no special skill to build, it's perhaps the most feared of the terrorists' nuclear choices. "They don't kill as many people," says Morton Bremer Maerli, a nuclear-terror expert at the Norwegian Institute of International Affairs, "but as a weapon of terror, they may be just as effective."
If there's reason for anxious Americans to feel hopeful, it's that pulling off a nuclear attack, even a low-grade one, is an enormously complicated business, and anything at all--from technical problems to supply problems to the simple dangers of fooling with radioactive material--could trip it up. For bin Laden, everything would have to go exactly right, or a nuclear strike wouldn't work. For the American military and the global law-enforcement forces arrayed against him, the job is to see to it that at least one of those things goes wrong.
--Reported by Hannah Bloch/Islamabad, Joshua Kucera and Violeta Simeonova/Sofia, Tim McGirk/Quetta, Andrew Purvis/Vienna, Douglas Waller/Washington and Yuri Zarakhovich/Moscow