THE PILOT TOOK OFF AT 0642 CDT, AND AT 0645, HE REPORTED LEVEL AT 4000 FT MSL. AT 0659:08, HE PREPORTED THAT HE WAS ENCOUNTERING ICING CONDITIONS AND REQUESTED A CLIMB TO 6000 FT. AT 0700:16, AFTER CHANGING FROM TRACON TO CENTER FREQUENCY, HE WAS CLEARED AS REQUESTED. AT 0700:37, THE PILOT REPORTED THAT HE THOUGHT HE WOULD HAVE TO DESCEND. THE CONTROLLER WAS UNABLE TO APPROVE A LOWER ALTITUDE ON COURSE, BUT CLEARED THE PILOT FOR A TURN (TO REVERSE COURSE) AND DESCEND TO 3000 FT. NO OTHER TRANSMISSIONS WERE RECEIVED FROM THE AIRCRAFT AFTER THAT. JUST PRIOR TO THE ACCIDENT, WITNESSES OBSERVED THE PLANE DESCENDING ON A NORTHERLY HEADING WITH THE WINGS ROCKING. THEY REPORTED THAT THE ENGINE WAS OPERATING. INITIAL IMPACT WAS WITH A TREE ON RISING TERRAIN. THE AIRCRAFT THEN STRUCK THE GROUND ON SLIGHTLY HIGHER TERRAIN, 16 FT BELOW AND 95 FT FROM THE INITIAL IMPACT POINT. THE WIND WAS STRONG FROM THE SOUTHWEST. PRIOR TO THE FLIGHT, THE PILOT WAS ADVISED OF THE POTENTIAL FOR ICING AND TURBULENCE.