Not jumping to any conclusion, merely a question as you can see: terrorism? I surely hope it isn't, but no one is speculating as to the cause.
Bridge corrosion coupled with a lot of weight perhaps?
Just now watching the railroad tanker car under a collapsed section, and knowing that AlQaida probably has as many graduate structural engineers as it does physicians, I’d say it was probably terrorism ~ but it didn’t quite come off the way they planned.