Nearly two years after the FBI effectively closed down air-taxi operator Sunjet Aviation, the federal criminal investigation into the October 1999 crash of a Learjet 35 that killed pro golfer Payne Stewart and five others has ended with no charges against Sunjet, the now-defunct company that operated the airplane, or its former chief pilot, Jim Watkins. In May last year, lack of causal factors forced the FAA to drop its attempt to revoke the pilot certificates of Watkins and other Sunjet pilots. The NTSB concluded in December 2000 that the probable cause of the crash was the incapacitation of the two pilots from lack of supplemental oxygen following a loss of cabin pressurization. But the Board was not able to determine what caused the cabin to depressurize or why the pilots were unable to receive supplemental oxygen.