At the time of 9/11 two notable technologies already existed: (1) Remote control airliners. Check out Jane's Aircraft from the year 2000. (2) Holographic color projectors. Then they were very expensive, somewhat experimental, and pretty much DoD territory. Now, they are used routinely in big shows in Asia and sometimes Australia.
Although I first thought remote controlled airliners were used, I'm coming around to the view that holographic projection was done.
