Still shouldn't have failed with no warning...
Very unusual for the steel to fail although that seems to be what everyone is focused on...I would think anything that would cause the steel to fail would be pretty obvious to a bridge inspector (like fissuers or Rust).
I almost wonder if the ground failed underneath one of the pillars...did they do proper Geologic studys of the baserock before they expanded the bridge from 2 lanes to 6 lanes? I thought I saw a picture where one appeared to be bent over like the leaning tower of Pisa. Is the water table lower now than when they built it?
Very strange incident. I hope they are able to narrow the cause with a reasonable degree of certainty.