As the acft (N21ST) was en route on a flt from Key West to Naples, FL, the plt of another acft saw a 'fire ball' in the vcnty of where N21ST subsequently crashed. When N21ST did not arrive, a search was initiated. The wreckage was fnd the next day at 1704 EST, in the Everglades Natl Park, nr Flamingo, FL. Drg impact, the main wreckage was buried in 30' of mud. The left outboard wing section (fm just outboard of the eng nacelle to the wing tip) was fnd aprx 1 mi fm the mainwreckage. An exam revealed the wing had failed where the nacelle fuel tank & the aux fuel tank boost pumps were mounted. No exhaust sys failure or leakage fm the fuel tanks was fnd in the area of the fire. The greatest fire dmg was at a point where the left nacelle fuel tank boost pump was mounted & aft fm there to where the rear wing spar had burned thru. The elec fuel boost pumps were not recovered after the acdnt. The ignition source for the fire was not determined.