enginerdfag here…FULL DISCLOSURE: Bridge-design is not my area of practice, and I have not reviewed the official design documents.
With that said, it's difficult to pinpoint why it failed without seeing the failure progression, however, the partially constructed structure was obviously not strong enough to support the weight + load test.
One of the first things they will do (probably) is review the official design documents (stamped plans, stamped calculations including any finite-element analyses) to determine if there was an engineering error.
If the engineering is sound, construction materials and/or techniques would be the next logical place to investigate (faulty rebar? incorrect concrete mix? etc.). Assuming they don't clean it all up and ship it to China, there should be plenty of material samples available.
If the engineering AND materials/techniques were sound, the next question (in my mind at least) would be "was there sabotage"?
and I agree with riggerfag…if it were MY project, I'd DEMAND testing be done in a manner that does not expose the public to harm.
hate to see shit like this. :(