AT 1205 BDT, THE PLT REPORTED DEPARTING RWY 20 ON AN IFR FLT. AT THAT TIME, THE BASES OF THE CLOUDS WERE REPORTED TO BE AT APRX 600 TO 700 FT WITH THE TOPS AT 3500 FT & ANOTHER CLOUD LAYER WAS ABOVE THAT. AT APRX 1207, THE ACFT CRASHED ON THE ARPT BOUNDRY, BESIDE THE WEST END OF RWY 9/27. THE IMPACT OCCURRED ON THE SOUTH BANK OF A SNOW COVERED, FROZEN RIVER. THE MAIN WRECKAGE CAME TO REST ON THE NORTH BANK AFTER TRAVELING APRX 300 FT. TWO WITNESSES HEARD WHAT SOUNDED TO THEM AS AEROBATIC MANEUVERS. ONE WITNESS STATED THAT HE SAW THE ACFT JUST BELOW THE BASE OF THE CLOUDS HEADING NORTH. ACCORDING TO HIM, THE ACFT STARTED A 360 DEG RIGHT TURN, MAKING A STEEP DESCENT DURING THE 1ST HALF OF THE TURN, THEN A STEEP CLIMB DURING THE 2ND HALF. HE REPORTED THAT AT THE COMPLETION OF THE TURN, THE ACFT'S NOSE DROPPED & THE ACFT ENTERED A DESCENT & CRASHED. ANOTHER WITNESS SAW THE PLANE DESCENDING BEFORE ITCRASHED. HE THOUGHT THAT IT HAD JUST COME OUT OF THE CLOUDS. VACUUM PUMP/ATTITUDE INDICATOR EXAMED, NO MALFUNCTION FOUND