Someone with a fairly sophisticated military. I DO NOT believe that Bin Laden could have pulled this off by himself.
As Post #20 by an engineer points out, the collapse of the building did not require great planning once you got an airliner into anywhere near the middle of the structure.
Reports from the passanger that managed to call on her cell phone reported that the highjaker's stabbed the stewardesses with knives and carpet cutting blades. These can be bought at any hardware store.
The planning consisted of finding four guys who could fly (not necessarily land) a plane once the pilots were stabbed, planning to board flights that took off at approximately the same time and smuggling carpet cutters past the X-Ray security agent.
I believe that this was the work of Osman bin Ladin but this was a pretty low-tech operation.
Such an attack can be prevented in the future with similarly low-tech solutions.
From now on, airline cockpit compartments must be armored, cockpit crews must be armed, the cockpits must locked before take-off and must not be opened under ANY circumstances even if every single passanger is threatened with death. In case of a bomb threat, the pilots must fly to the nearest body of water or unpopulated area and circle since, if you are going to die anyway, it is much nobler to die protecting others on the ground. As the Israelis do, plain-clothed, armed security must be aboard every flight to shoot it out with the bad guys in the passanger compartment.
And, as Politically Incorrect as it is (or was until this morning), there is a value in racial profiling. If you look Middle Eastern, you must expect that special attention will be given to you by security.