People jump from the Skyway Bridge quite often and the radio stations joke about it. The people at WTC didn't go to work for their lives to abruptly end. The Homeland Security can give me alerts any time they want to - we must still all be careful. Our borders are still too porous and except maybe for lack of funds, I am willing to bet there are still dangerous sleeper cells out there who want to do us harm. Loss of life means nothing to them.
If you saw this in person, I pray that your worst memories of that day soften so that you can feel some inner peace.