1. Accidents happen. If the original engineering firm followed accepted practices then it doesn't matter how quickly someone else found the flaw after the flaw was made evident by the collapse.

2. "After you've dropped a bridge...": Until the investigation is complete, no one has any business assigning responsibility. (In fact, the investigation probably won't assign responsibility; that will be the job of the court system in the subsequent lawsuits.)

http://to70.com/why-aviation-safety-experts-dont-talk-about-...