At 1:47 p.m., the north end of the installed bridge span sagged deeply, then fractured at the first diagonals, folded, and immediately dropped the heavy full span onto the roadway below. A surveillance video shows the collapse sequence took only a few video frames.

A witness reported that at the moment of collapse, a blue box fell loose from the hook of a crane. His impression was that its fall triggered the collapse. It dropped onto the roof of the bridge next to workers, and where the roof and span broke apart.

Post-collapse photos of the bridge show a post-tensioning rod jutting out about 6 feet from the destroyed blister, with a blue-colored hydraulic jack still attached.

The span that collapsed weighed 950 short tons (861 tonnes).

At the time of the collapse, the roadway was open and there were multiple cars stopped at a traffic light under the span. Eight cars are reported to have been crushed. Six people were killed and nine others were injured; while early reports said that the death toll was expected to rise, authorities said on March 18 that all bodies had been accounted for. One construction worker was killed and two others were hospitalized; an FIU student was also presumed dead.

More >  https://en.wikipedia.org/wiki/Florida_International_University_pedestrian_bridge_collapse