Terrorists snuck in airport security and took control, or hi-jacked, 4 planes. Two hit the world trade center which killed many people. After time, the towers collapsed killing the police and firemen by the towers. The other airplane collapsed into the pentagon, the US's defense head quarters. The last one crashed in pennsylvania killing all of those passengers. The passengers were brave enough to stop the plane. People think that the 4th plane was going to collide with the capitol.