I don't quiet know what they were trying to show. When it comes to violence like that where lots of innocent people died, there's no logical reasoning behind killing people. One could explain it as they wanted to cause panic and fear to the western world or they wanted to flex their muscles and show that the US was not as safe as we thought we were. Radicals do things just do it.
The hijackers took control of several planes I believe and were targeting DC but instead hit the pentagon and the twin towers and then one crashed in a field in Pennsylvania. The people on the plane from what I heard struggled with the hijackers and because of that the hijackers weren't successful in targeting DC but lots of people died, about 3,000 and some have died recently from the outcome of the attack.
