The truth about 9/11?

Okay i am really confused. I was watching Flight 93 on youtube and noticed that a lot of the people were saying it was the U.S. goverment that planned the whole attack. They said that since there was no parts of the plane 93 and the plane that hit the pentagon there must have never been a plane. Do you think it was the goverment who did this or terriost and explain your answer!