THE PILOT TOOK OFF IN FOG AND LIGHT RAIN AND CLIMBED INTO CLOUDS AT ABOUT 300-400 FEET ABOVE THE GROUND WHILE BOTH LANDING LIGHTS WERE EXTENDED AND ILLUMINATED. ALSO THE AIRPORT'S ROTATING BEACON (LOCATED AOBUT A MILE FROM THE ACCIDENT SITE) WAS FLASHING ON THE CLOUDS ACCORDING TO A GROUND WITNESS. JUST AFTER ENTERING THE CLOUDS THE PILOT WAS ISSUED A LEFT TURN FROM HIS PRESENT HEADING OF 350 DEGREES TO A HEADING OF 270 DEGREES AND TOLD TO CONTACT DEPARTURE CONTROL. HE DID NOT MAKE THIS CONTACT AND HE CONTINUED THE LEFT TURN (NOW DESCENDING) TO A HEADING OF 210 DEGREES AT WHICH TIME THE AIRCRAFT COLLIDED WITH TREES AND A LARGE ELECTRICAL TRANSMISSION LINE AND THEN THE GROUND. THE ENGINES WERE OPERATING AT A HIGH POWER SETTING AT THE TIME OF IMPACT. ALL BROKEN FLIGHT CONTROL CABLES REVEALED EVIDENCE OF OVERSTRESS SEPARATION. NO EVIDENCE OF PREIMPACT MECHANICAL FAILURE OR MALFUNCTION WAS FOUND.