THE ACFT COLLIDED WITH TREES & GROUND DURING IMC WX AT NIGHT. THE PLT OBTAINED A WX BRIEFING AT 1731 HRS CDT BY PHONE FROM HIBBING FSS. HE WAS ADVISED THAT VFR FLT WAS NOT RECOMMENDED. NO OTHER WX BRIEFING FOR THIS FLT WAS RECORDED. WITNESSES AT THE ARPT OF DEPARTURE SAID THE WX WAS DETERIORATING AS THE PLT PREPARED TO LEAVE. HE SAID HE COULD POP THRUTHE LOW SCUD WITH NO PROBLEM. THE ARPT MGR CALLED HIM ON UNICOM WHEN SHE HEARD THE ENG START TO ASK HIM IF HE WAS LEAVING. HE SAID HE WAS BECAUSE THE WX WAS FORECAST TO WORSEN BY MORNING. HE DEPARTED AT ABOUT 2145 HRS EDT ON RWY 16. THE PLT WAS NOT INSTRUMENT RATED. THE ACFT WAS OBSERVED BY WITNESSES FOR 20-30 SECONDS AFTER TAKEOFF AND WAS SEEN MAKINGA SLIGHT TURN TO THE SW. THIS WAS THE LAST TIME THE ACFT WAS SEEN IN FLT. THE WRECKAGE WAS NOT LOCATED FOR 30 DAYS. THE ARPT MGR LATER STATED THAT THE VISIBILITY AT THE ARPT THAT NIGHT WAS LIMITED TO 1500 FT DUE TO GROUND FOG. ANOTHER PLT TERMINATED HIS FLT DUE TO THE FOG AND SAID THAT THE AREA HAD BEEN ABOUT 50 PERCENT COVERED BY FOG.