NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Contract No. DAAD19-02-C-0049 between the National Academy of Sciences and the Department of the Army. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organization that provided support for the project.

Cover: The Pentagon burning after being struck by a hijacked commercial airliner, September 11, 2001. Courtesy of Reza Marvashti, The Free Lance-Star, Fredericksburg, Virginia.

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