It only became clear with investigations in the months after Katrina that flooding in the majority of the city was not directly due to the storm being more powerful than the city's defenses. Rather, it was caused by what investigators termed "the costliest engineering mistake in American history". The United States Army Corps of Engineers designed the levee and floodwall system incorrectly, and contractors failed to build the system in places to the requirements of the Corps of Engineers' contracts. The Orleans Levee Board made only minimal perfunctory efforts in their assigned task of inspecting the city's vital defenses. Legal investigations of criminal negligence are pending.