Here is the link to the chapter in the 9/11 Commission's Report on the hijacking of the planes. It's sobering reading. The chapter is titled "'We Have Some Planes.'"
For me, two sentences stood out about United Flight 93:
"Jarrah's [The hijacker's] objective was to crash his airliner into symbols of the American Republic, the Capitol or the White House. He was defeated by the alerted, unarmed passengers of United 93."