DURING THE 0910 TELEPHONE WX BRIEFING BY THE ST. LOUIS FSS, THE PLT MENTIONED THAT THERE WAS GROUND FOG IN THE SALEM AREA. AT ABOUT 1010 THE PLT DEPARTED FROM RWY 16, MADE A LEFT TURN & CROSSED THE ARPT HEADING NORTH. A WITNESS WHO DROVETHE PLT TO THE ARPT STATED HE WATCHED THE ACFT UNTIL IT WAS OBSCURED BY CLOUDS AS IT CLIMBED. ANOTHER WITNESS, WHO WAS LOCATED ABOUT 1/2 MI EAST OF THE APPROACH END OF RWY 16, STATED HE SAW THE ACFT FLYING NORTH BEFORE IT WAS OBSCURED BY A NEARBY BUILDING. HE THEN HEARD THE ENG REV UP, THEN A CRASH. HE REPORTED THAT THE SKY WAS OVERCAST & GROUND VISIBILITYWAS ABOUT A MILE. WRECKAGE EXAMINATION DISCLOSED THAT THE LEFT OUTER WING SEPARATED AT THE INBOARD EDGE OF THE AILERON. THE EMPENNAGE DISPLAYED EVIDENCE OF LEFT WING IMPACT FORWARD OF THE VERTICAL STABILIZER. THE SPARS & WING SKIN DISPLAYED EVIDENCE OF OVERLOAD FAILURE. THE 0958 WX AT VICHY,MO, LOCATED 34 NM NW OF SALEM, WAS 2 MI IN FOG & HAZE.