Joking on twin towers is like joking about god. Both do not exist, but everybody are angry when you say it
Are you seriously making fun of the 9/11 attacks? You think it is just a joke, but it isn't for some people.
It is not a joke, but some people take it as one. I don't find it funny, but I won't care about your feelings if you do.
A lot of people died during the 9/11 attacks so I do not think that it is of your best interest to joke about it. Have you watched Zero Dark Thirty? It does make you see what are the things that people have experienced that day so I recommend you take a look on that one so that you wouldn't make a joke about it.