Like all major disasters, natural or manmade, the terrorist attacks of September 11 resulted in a tragic loss of life and destruction of property, as well as short-term disruption of economic activity. In addition, because of the size and premeditated nature of the attack, there are likely to be more lasting effects in some industries and segments of the U.S. economy. Conceptually, the economic costs to the United States stemming from the 9/11 terrorist attacks can be broken down into several categories, largely depending on their nature (direct and indirect) and on the time period examined (immediate, short-term, medium-term and long-term). Those costs that are short run and direct are clearly the easiest to identify and measure. Estimates covering longer periods of time and focused mainly on indirect costs require numerous assumptions concerning counterfactuals and hence are on less firm ground.
Naval Postgraduate School, Center for Contemporary Conflict: http://www.ccc.nps.navy.mil
Strategic Insights (August 2002), v.1 no.6