There were many phone calls placed to and received from the passengers aboard with some passengers describing in detail the events they were witnessing. One person listened to the open telephone all the way to when it crashed into the ground. There is also the cockpit voice recording that has been transcribed and played for the families to listen to which details what was occurring in the cockpit. The flight data recorder recorded all of the input to the flight controls and all the buttons that were being pushed and the switches that were being turned. So actually quite a bit more is known about the events aboard United Flight 91 than you say.
Well put.