WOW, the bridge was the one I used almost every day. I lived at 1 mile from the bridge. The steel truss of the bridge are twisted like plastic truss. What was the temperature? I suspect the increased temperature could have weakened the supporting truss, which might have expanded it to displace beyond its safety zone at one of its support. The disastrous expansion could be the joint result of air temperature, stress and vibration. Because one of the lane was closed at that time, all rush-hour vehicles must be running through a narrow lane causing unbalance in the loads between the two parallel girders. In such a case, there is a chance to develop extra lateral stress on top of stress due to the over-crowded movement of vehicles, and the vibration therein resulting heating up of the steel structure. Once it starts softening of only a fraction of the giant girder, the whole structure start losing its stiffness in geometric order bringing the entire structure into collapse.