Originally Posted by MarkyX
We can rule out a heat-seeking missile...
According to the FDR from UA93, both engines were running normally (and attached to the plane) at 70% thrust at the moment of impact.
The NTSB FDR report for UA93 is here
For laymen, someone on a professional pilot's forum has provided the key specs at impact here
According to the Flight Data Recorder from UA93, it impacted the ground at about 500kts (575 MPH) in a 40 degree nose down attitude, with a 150 degree roll angle to starboard.