AT ABOUT 0401 EST, THE PILOT TOOK OFF ON AN IFR FLIGHT AND WAS CLEARED TO CLIMB AND MAINTAIN 6000 FT MSL. RADAR CONTACT WAS ESTABLISHED AT ABOUT 0402. AFTER ABOUT ANOTHER 30 SECONDS, RADAR CONTACT WAS LOST. THE AIRCRAFT IMPACTED ABOUT 3 TO 4 MI FROM THE DEPARTURE RUNWAY IN A STEEP NOSE DOWN ATTITUDE. TO THE EXTENT POSSIBLE, AN EXAMINATION OF THE ENGINE AND STRUCTURE WERE MADE, BUT NO PREIMPACT/MECHANICAL MALFUNCTIONS OR FAILURES WERE FOUND. SEVERAL WITNESSES REPORTED THAT THE ENGINES WERE OPERATING AT HIGH POWER UNTIL IMPACT. THE TOTAL CARGO LOAD (BAGS AND BOXES) WAS ESTIMATED TO BE 399 LBS. NO CARGO NET WAS FOUND IN THE WRECKAGE, BUT SEVERAL TIEDOWN ROPES WERE FOUND. THE POSSIBILITY OF THE CARGO SHIFTING WAS CONSIDERED, BUT WAS NOT DETERMINED. IF THE CARGO HAD SHIFTEDFULL AFT DURING THE CLIMB, IT WOULD HAVE BEEN ONLY ABOUT .5% BEHIND THE AFT CG LIMIT.