AT ABOUT 1605 CDT, THE PILOT ACTIVATED HIS VFR FLIGHT PLAN FROM NASHVILLE, TN TO VENICE, FL. AT 1813, THE PILOT CONTACTED TALLAHASEE FSS AND WAS BRIEFED ON THE WEATHER CONDITIONS TO SARASOTA, FL. THIS WAS THE LAST KNOWN RADIO COMMUNICATION WITH THE AIRCRAFT. A RADAR CONTROLLER AT JACKSONVILLE CENTER OBSERVED A TRACK ALONG AIRWAY V-97 THAT MATCHED THE PILOT'S INTENDED ROUTE. HOWEVER, THE PILOT WAS NOT IN RADIO CONTACT WITH JACKSONVILLE CENTER. LATER, A COMPUTER READOUT OF THE RADAR INFORMATION SHOWED THAT THE FLIGHT PATH AND ALTITUDE OF THE ASSUMED AIRCRAFT BECAME ERRATIC AS IT CONTINUED OVER THE GULF OF MEXICO. IT CONTINUED TO THE VICINITY OF LICKS INTERSECTION WHERE IT DISAPPEARED FROM THE RADAR IN AN AREA OF THUNDERSTORM ACTIVITY. THE CAP INITIATED A SEARCH WHICH WAS CONTINUED UNTIL 7/11/82 WITH NEGATIVE RESULTS. THE AIRCRAFT WAS PRESUMED TO HAVE BEEN DESTROYED AND THE OCCUPANTS WERE PRESUMED DECEASED.