A pedestrian bridge near Marcy, New York collapsed during construction on October 12, 2002, injuring nine workers and killing one. The authors were retained to investigate the reasons for the bridge collapse, and whether the actions or omissions by the key players contributed. The pedestrian bridge was designed as a straight, single-span, composite box girder composed of a trapezoidal steel tub with stay-in-place forms and concrete deck. The collapse occurred while the concrete deck was being placed. Based on eyewitness accounts and the position and orientation of the debris, the mode of failure was consistent with lateraltorsional buckling of the entire girder cross-section. While conventional engineering training would preclude lateral torsional buckling of a beam bent about its weak axis as a potential failure mode, computer simulations using nonlinear analysis and eigenvalue buckling analysis confirm eyewitness accounts, that is, a point of bifurcation is reached during concrete placement on the torsionally flexible steel tub girder. Since this failure, which was slightly predated by a very similar failure in Sweden, the State of New York and AASHTO have appended bridge design specifications to explicitly require diagonal bracing between the top flanges in order to suppress this buckling mode. A discussion of the collapse and the failure mode follow, highlighting the lessons that the engineering community has learned from the collapse. Copyright ASCE 2009.