THE PLT & PASSENGER DEPARTED ON A LATE NIGHT FLT FROM FORT LAUDERDALE TO KEY WEST, FLORIDA. THE FLT PROCEEDED WITHOUT APPARENT INCIDENT UNTIL THE ACFT WAS IN AN OVERWATER DESCENT OVER THE GULF OF MEXICO. THE DESCENT WAS NORMAL UNTIL ABOUT 0223 EDT, WHEN THE ACFT WAS APRX 34 MI NORTHEAST OF KEY WEST. RADAR DATA SHOWED THAT AT ABOUT THAT TIME, THE ACFT'S HEADING BEGAN TO CONSTANTLY CHANGE, WHICH RESULTED IN THE FLT MANEUVERING IN CLIMBS & DESCENTS OVER A 4 MI AREA. AFTER MANEUVERING ABOUT 4 MIN, CONTACT WITH THE ACFT WAS LOST, AND SEARCH & RESCUE EFFORTS WERE INITIATED. AT 0655 EDT, AN OIL SLICK WAS FND IN THE VCNTY OF THE ACFT'S LAST KNOWN POSITION. AT ABOUT 0900 EDT, A COAST GUARD SHIP RECOVERED DEBRIS FROM THE ACFT, BUT THE ACFT'S MAIN WRECKAGE & THE OCCUPANTS WERE NOT FOUND. THE ACFT WAS PRESUMED TO HAVE BEEN DESTROYED & THE OCCUPANTS WERE PRESUMED TO HAVE BEEN FATALLY INJURED OR DROWNED. THE WX AT KEY WEST WAS IN PART: 1500' SCATTERED, VIS 6 MI WITH HAZE. THE WX AT THE ACDNT SITE WAS NOT VERIFIED.