The terrorist attacks in New York and the Pentagon could have been easily prevented. The entire system was slacked do to the confidence in the people of the united states , which in part is do to the current generation being born after pearl harbor ,which was the last attack on the united states that can even comes close to the magnitude of the September 11 attack.
First of all the airframe should have been designed differently, the airframe is everything but the engines. The front of the airplane should be separated by a bulkhead so that entry into the cockpit can only be accessed from outside the plane. The drawbacks to the airlines would be costly for the duplicate crew that would have to be on each plane for unforeseen emergencies and of course the cost of modifying all airplanes. The new costs would definitely put many businesses under, but the job losses would be absorbed within the other fields repairing the problem.
Another foreseen cost would be the loss of seats for passengers so as to give the crew enough room to be comfortable. The costs overall are nothing compared to the lives that could be potentially saved if the F.A.A. would make the separation in airplanes mandatory in all new planes.
Secondly the response time that the military had that day was les than adequate for the world's top technically advanced military. The United States has the resources to monitor all air traffic and respond within five minutes of any course changes made that would come into contact with restricted airspace. The military's resources could keep planes in the sky at all times and across the country so an instant response would at least be a possibility. The computers that are available for military use could easily do most of the monitoring and alert personal so as to check to data and make a decision. A separate branch of the combined F.A.A. and the air force could take the task on so other branches of government wouldn't be diminished.
Thirdly the question remains "how did the terrorists get weapons onboard the aircraft." The airports have metal detectors and security personal everywhere so much so that even an honest citizen feels watched like the old joke says don't say hi to your friend Jack at an airport. The oversights by the airports in paying low wages and having even lower standards of hiring have led to commercial airlines being used for drug smuggling and weapons transport. Dogs and more efficient personal and equipment should and easily could be used at all air ports. To have exceptional transportation you must take exceptional measures to secure it from people who can only see the devastation it could be used for.
To conclude the aircraft manufacturers, the airlines, and the airports all can be accountable for what happened on September 11, 2001 not to the extent of being equal with the terrorists but as a comparison in the judicial system it would be like first degree murder and involuntary manslaughter. The money grubbing executives watched the all mighty buck instead of foreseeing the obvious future progression of airline hostage situations.