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KNOXVILLE, Tenn. — Loss of cabin pressure alone is not enough to explain Monday’s jet crash that killed golfer Payne Stewart, a University of Tennessee engineering professor said Thursday.

Stewart’s plane apparently flew pilotless for about four hours before crashing into a South Dakota field.

Investigators are looking for valves and parts of doors, windows and hydraulic components that could indicate a rapid loss of pressure.

But Dr. Mancil Milligan, a pilot and UT-Knoxville aerospace engineer, said problems other than pressure loss probably contributed to the crash.

“A valve could have failed, a window could have cracked, a door seal could have blown, but none of those things alone could explain the whole sequence of events,” Milligan said. “There are alarms and safety systems to handle the situation in any of these failures. Something incapacitated the crew other than oxygen starvation.

“The oxygen system may have malfunctioned, oxygen masks may not have deployed, or toxic fumes from a fire on the plane might have overcome them.”

Milligan said those aboard a plane can lose consciousness quickly from lack of oxygen if a door or window seal breaks at high altitude. Pilots are trained to recognize and react to this, and having two pilots made it less likely on Stewart’s jet, he said.

“The plane warns the crew that it is losing pressure,” Milligan said. “The quick-donning oxygen masks are right there in their faces. How to react to rapid depressurization is part of their training.”