THE PLT RECEIVED A WX BRIEFING AND FILED AN IFR FLT PLAN TO BOEING FIELD, SEATTLE, WA. HE DEPARTED KALISPELL, MT, AT 1135 MST. WHEN THE PLT ARRIVED IN THE SPOKANE AREA, HE OBTAINED WX REPORTS FOR SEATTLE AND DECIDED TO TURN AROUND AND RETURN TO KALISPELL. WX AT KALISPELL HAD DETERIORATED. THE PLT WAS CLEARED FOR A CONTACT APCH. THE PLT ADVISED THE SALT LAKE ARTCC THAT HE HAD A SMALL LAKE IN SIGHT BELOW, WAS CANCELLING IFR AND PROCEEDING VFR. ICE FISHERMEN OBSERVED THE ACFT MAKE SEVERAL PASSES OVER THE LAKE THEN HEARD THE SOUND OF IMPACT. THEY DESCRIBED THE WX AS LOW CEILING, REDUCED VISIBILITY, SNOW, AND FOG. ALTHOUGH THE PLT COULD NOT RECALL THE EVENTS LEADING UP TO THE ACCIDENT, HE SURMISED THAT HE MAY HAVE ATTEMPTED A PRECAUTIONARY LANDING DUE TO AN ACCUMULATION OF AIRFRAME ICE. FLT PRECAUTIONS FOR MODERATE RIME ICE IN CLOUDS AND PRECIPITATION FROM FREEZING LEVEL TO 14,000 FT HAD BEEN FORECAST.