One thing the article doesn't address is that some time after the UAL23 cabin had been locked a couple of people were seen walking the aircraft's aisle, when the aircraft was opened up the aisle hatch going down into the avionics bay was found lying in the aisle, indicated that some one had entered the aircraft via the external hatch located on the aircraft's belly behind the nose wheel to the avionics bay (statement made by UAL23 Captain in the documentary).
Another thing he mentioned was that normally this particular flight taxied uninterrupted from the gate to the runway and took off, there being no traffic to cause a stop and delay, on this particular occasion there were a line of aircraft in front delaying take off, had there not been that traffic in front they would have been airborne prior to the tower closing operations down.
Documentary available here, free registration.
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