THE ACFT COLLIDED WITH TREES & TERRAIN AS THE PLT WAS ATTEMPTING TO REACH AN ARPT. HE HAD TOLD ATC PSNL THAT STRUCTURAL ICE WAS ACCUMULATING ON THE ACFT AND THAT HE WAS UNABLE TO MAINTAIN ALT. SUBSEQUENTLY, THE ACFT DSCNDD BELOW AN ALT WHERE RADAR CONTACT COULD NOT BE MAINTAINED. AIR & GROUND SEARCH PROCEDURES WERE STARTED AFTER ALL CONTACT WITH THE ACFT WAS LOST. THREE DAYS LATER, THE ACFT WAS FOUND BY GROUND SEARCHERS. THE WEATHER ALONG THE ROUTE CONSISTED OF LOW OVERCAST SKIES, BELOW FREEZING TEMPERATURES & FOG. LAYERED PIECES OF ICE WERE FOUND IN THE WRECKAGE AREA, WHICH WERE 3/4 TO 1-1/4 INCH THICK; THESE PIECES OF ICE CONTAINED INNER SURFACES THAT MATCHED THE SHAPE OF AIRFOILS. WEATHER IN THE AREA WAS INFLUENCED BY A WARM FRONT WITH LOW OVERCAST SKIES, FREEZING TEMPS & FOG TO THE NORTH OF THE FRONT. BEFORE THE FLT, THE PLT WAS ADVISED OF OCCASIONAL MODERATE RIME ICING ALONG HIS ROUTE.