<http://www.cnn.com/2004/US/12/27/rumsfeld.flt93/>
But a lot of the evidence I've read, along with eyewitness testimony from that area in PA on 9/11, suggests that the plane simply did not crash, that it was coming apart before it hit the ground. But who really knows.
If you haven't heard the air traffiic control tape from United 93, you should check it out:
<http://69.57.136.18/cvrfiles2/ua93.wav>
Pretty creepy. And you can hear one of the hijackers speaking, though he thinks he's talking to the passengers, when in fact he's transmitting to controllers and other pilots.
Dennis