What happened
During a climb following takeoff, the crew encountered an issue where the cabin would not pressurize. The first officer began leveling off at 16,000 feet; however, the captain ordered him to continue the climb to the assigned flight level of FL330 while the captain moved aft into the aircraft to locate the source of the problem.
The captain left the cockpit carrying a portable "on demand" oxygen system with a 15-minute supply. After the captain failed to return to the cockpit, the first officer attempted to signal him. Although reluctant to countermand the captain's previous order, the first officer performed a series of descents to 13,000 feet. Approximately 30 minutes later, the first officer left the cockpit and discovered the captain unconscious and unresponsive in the forward cargo area. The captain had an oxygen mask on his face, and his foot was entangled in a cargo net covering a pallet.
The first officer attempted to revive the captain without success, subsequently declaring an emergency and landing at Lubbock. The captain was rushed to a hospital but was dead on arrival. There was 1 fatality.
The investigation
An examination of the aircraft revealed that the aft pressure bulkhead had not been installed, having been removed for maintenance prior to the flight. Investigators also examined the portable oxygen system and found it was still full, indicating the captain had little to no use of the oxygen. The system was tested and found to be functioning normally. A group of eight forensic pathologists concluded that the captain died from hypoxic hypoxia.