>Where the systems of security of the United States were?

Short of strip searching everyone who boarded those planes, there really is nothing that could have prevented the hijackings. All you have to do is have the training and the mentality to be willing to kill yourself in an act of terrorism. All those bastards had to do was train, get a knife through security, and break into the cockpit and kill the pilots.

The knifes do not have to be made of metal to be deadly. There are other materials that can be made into knives that will pass by a metal detector.