美国 “国家交通安全管理局”花了18个月的时间来调查这场事故，最后得出的结论是，由于这座桥常年服务超载的车辆，最终因无法经得起超负荷而倒塌。

The NTSB cited a design flaw as the likely cause of the collapse, and asserted that additional weight on the bridge at the time of the collapse contributed to the catastrophic failure.[6]

On November 13, 2008, the NTSB released the findings of its investigation. The primary cause was the under-sized gusset plates, at 0.5 inches (13 mm) thick. Contributing to that design or construction error was the fact that 2 inches (51 mm) of concrete were added to the road surface over the years, increasing the dead load by 20%. Also contributing was the extraordinary weight of construction equipment and material resting on the bridge just above its weakest point at the time of the collapse. That load was estimated at 578,000 pounds (262,000 kg) consisting of sand, water, and vehicles. The NTSB determined that corrosion was not a significant factor, but that inspectors did not routinely check that safety features were functional.[136]