Sept. 11 was terrifying for one main reason: We had little idea about al Qaeda's capabilities. It was a very reasonable assumption that other al Qaeda cells were operating in the United States and that any day might bring follow-on attacks. (Especially given the group's reputation for one-two attacks.) We still remember our first flight after 9/11, looking at our fellow passengers, planning what we would do if one of them moved. Every time a passenger vis