The black box was recovered and the families were allowed to listen to the tapes. They speculated details based on what was heard on the tapes of the black box. They heard the hijackers, they heard the refreshment cart ramming into the door of the cockpit and lots of shouting, both in Arabic and English. Between the cell phone calls made and those tapes, they have been able to piece it together pretty thoroughly.
ETA: You can read the transcript of the cockpit tape
here
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