There was also a commision at some point in time to determine why the internal structure of the Towers and the fire-proofing on those internal structures failed.
I heard in an interview with the architect of the twin towers that he feels personally responsible for what happened...
Because most buildings are designed to withstand commercial jets flying into them and it's all his fault for being a slacker.
Figured I'd add something to this. The Towers were designed to survive an impact from a commercial jet. However, the assumption was that it would be an accidental impact (like the B-17 that hit the Empire State building). There's several key points to remember:
Jet liners weren't as large back when they designed the building. The jets that hit the WTC towers were about 20% more massive then what they had accounted for.
The assumption was that a plane that got lost in fog or something, would likely be traveling slowly, not at near top speed.
There was no assumption that an accidental crash would involve jets with nearly full fuel tanks. A plane is extremely unlikely to get lost taking off from NY, go around in a circle with a full fuel tank, and hit the WTC. The scenario that was planned for was a commercial plane arriving in NY, going off course and hitting one of the towers. In that case, the fuel tanks would be low, since airlines only put just a bit more fuel in a plane then what it would take to get to where they are going (less weight means more money saved).
Those three factors were quite reasonable assumptions to make in 1971, when the towers were constructed. There's some argument over whether it's possible to make a building that tall that could have survived the impact of large commercial jets flying at near full speed with full fuel tanks.
While the architect has admitted full "fault" for the buildings not surviving the impact, I don't think anyone has tried to make a case that he could have built them in a way that would have allowed them to survive. The conditions of those hits were just too extreme.