The New York Times has a report on the audio tapes of what happened on Flight 93 on September 11 ,2001, played at the Moussaoui trial for the first time in public. It's hard to do justice to the event or the tapes with just an excerpt, but here is the most chilling of the very chilling parts:

  The recording ends with a three-minute crescendo of noise as a passenger apparently just outside the door shouts: "In the cockpit! If we don't, we'll die!"
  On the other side of the door, two hijackers are heard deliberating before deciding to end the flight to avoid being overcome.  "Is that it? I mean, shall we pull it down?" one asks in Arabic and the reply is, "Yes, put it in it and pull it down." They then both scream repeatedly "Allah is the greatest" in Arabic as the planes goes down at 10:03 a.m. into a field in Shanksville, Pa., at more than 500 miles an hour. Aboard were 33 passengers, 5 flight attendants, 2 pilots and the 4 hijackers.

The story adds, a few sentences later: "Mr. Moussaoui, who was in jail in Minnesota at the time of the attacks, smiled broadly at times during the playing of the recording."