The plane carrying U.S. Open champion Stewart to Dallas took off from Orlando at 9.19 a.m. but stopped responding to air traffic controllers after 9.45 a.m. That prompted them to call the Air Force, which routinely tracks out-of-control aircraft. Although such planes can be shot down if they present a threat to public safety, that option was never considered in the case of Stewart's plane. Though the Learjet is equipped with masks and other depressurization emergency equipment similar to that of an airliner, some experts believe that insidious rather than sudden depressurization can confuse and disorient a pilot to the point that he or she would be unable to react. Others insist that the warning systems would have alerted the pilot to a slow leak, making an explosive depressurization a more likely scenario. "All we can say for certain right now," says Hannifin, "is that this incident may well have been unique in aviation history."
The mystery of Payne Stewart's last flight may sound like "X-Files" material, but investigators believe a simple case of depressurization may have killed the star golfer and his five fellow travelers. The miracle is that the only victims in Monday’s tragedy were the six people aboard the Learjet that was carrying Stewart to a Texas tournament. "Once its crew were incapacitated, that plane was like an artillery shell crossing at least a dozen busy air routes at 400 or 500 miles an hour from Florida to the Dakotas," says TIME aviation correspondent Jerry Hannifin. "It was an extremely dangerous situation that could have caused a terrible accident if it hadn’t been so well managed by air traffic control." National Transportation Safety Board officials began on Monday to sort through the wreckage of the crash, which occurred when the Learjet ran out of fuel and slammed into a South Dakota field. An Air Force pilot sent up to chaperone the doomed craft saw the internal windows frosted over, suggesting that the Learjet's cabin had been depressurized – which could have suffocated the four passengers and two pilots.