Rolf Mowatt-Larssen is paid to think about the unthinkable. As the Energy Department's director of intelligence, he's responsible for gathering information about the threat that a terrorist group will attack America with a nuclear weapon.
With his shock of white hair and piercing eyes, Mowatt-Larssen looks like a man who has seen a ghost. And when you listen to a version of the briefing he has been giving recently to President Bush and other top officials, you begin to understand why. He is convinced that al-Qaeda is trying to acquire a nuclear bomb that will leave the ultimate terrorist signature -- a mushroom cloud.