What happened on September 11 that shocked Americans?

There was a terrorist attack. Commercial airliners were hijacked, and used to crash into the World Trade Center. Two attacks took down both towers of the center.

A third plane was hijacked with plans of crashing into the Pentagon, but a few passengers fought back against the terrorists. They managed to crash the plane in an area that only took the lives of those on board.

There were 3,000 deaths and 6,000 injuries due to the attacks.