Review of Studies of the Economic Impact of the September 11, 2001, Terrorist Attacks on the World Trade Center [open pdf - 304KB]
"The terrorist attacks caused enormous destruction in New York City. The losses included the direct costs of the destruction to lives and property, as well as cleanup, and the indirect costs of lost income brought about by business closings and related spending reductions. It is expected that some of the losses will be covered by payments from private insurance, emergency federal relief funds, and charitable contributions. Other losses, however, may never be recovered because some individuals and businesses may not have been insured or may not qualify for federal relief or charity."
Government Accountability Office (GAO): http://www.gao.gov/