Najibullah Zazi after his arrest in a terrorism investigation
Eight years after 9/11, the specter of terrorism still haunts the United States. Just last week, F.B.I. agents were working double time to unravel the alarming case of a Denver airport shuttle driver accused of training with explosives in Pakistan and buying bomb-making chemicals. In Dallas, a young Jordanian was charged with trying to blow up a skyscraper; in Springfield, Ill., a prison parolee was arrested for trying to attack the local federal building. Meanwhile, the Obama administration struggled to decide whether sending many more troops to Afghanistan would be the best way to forestall a future attack.