terrorist attacks in the United States

In the United States, there have been a few terrorist attacks. The most recent major one occured on September 11, 2001. Four planes were hijacked and three crashed into heavily populated buildings. That day many people were affected. The motives of the terrorists are probably to set the world right as they view it. Todaay, we as a nation have come great lenghts to prevent these attacks. One way is heavy security at airports. Hopefully there will not be terrorist attacks in the future