There is an interesting story of an inept attempt to assasinate Nixon. The fact that someone wanted to kill Nixon is neither surprising nor important.

What is most interesting is the fact that the means of destruction used on September 11 had been tried before, and every Federal agency responsible for preventing such acts knew it.
It doesn't matter that the attempt was not publicized at the time. Due to poor execution, Nixon was never in danger from that attack, and he was already on his way to political self-destruction anyway.

It was, however, well known to the Secret Service, FBI, FAA and the military. It could not take much imagination to know that with better skill and planning, someone could use a hijacked plane to destroy any structure, and that, if the hijacker were in control of a plane in the air, it would take fast military action to prevent it.

The excuse that such a crime would be unthinkable or even unlikely can no longer be considered valid for the poor preparedness and confusion that allowed the September 11 attacks to succeed.

It was obvious that any airplane, especially a commercial airliner, is potentially a deadly weapon. Why, then, was it possible to turn off the transponder? Why was radar inadequete to track a large plane gone off course? Why did the military not have a plan in place to counter just such an attack as had already been attempted? Were they counting on every future terrorist to be so stupid as to try to hijack the plane on the ground and shoot the pilot and co-pilot without knowing how to fly a plane?

What has been done to correct the technical vulnerabilities? Passenger screening is better, but if that fails, or a private plane were used, are we prepared to detect and prevent another attack?