It was the gusset plates!

Source: Design flaw caused bridge collapse

Excerpt:

The official, who was briefed by the National Transportation Safety Board, said that investigators found a design flaw in the bridge's gusset plates, which are the steel plates that tie steel beams together.

The official spoke on the condition of anonymity so as not to pre-empt an update being provided later Tuesday by the NTSB chairman, Mark V. Rosenker.


Comment: This was an early-on theory. See Problems with the gusset joints?