AFTER RTRNG FM A FLT, THE PLT TOOK OFF AGAIN (USING RWY 28L) & TURNED TO THE SE. A WITNESS (SE OF THE ARPT), SAID HE SAWTHE ACFT AT LOW ALT (ESTD 100 TO 200 FT) FLYING IN HIS DRCTN. HE NOTED NO OTR INDCN OF A PROBLEM UNTIL AFTER THE ACFT MADE A 'VERY SHARP BANKING TURN TO THE LEFT' & REVERSED DRCTN BACK TO THE ARPT. THE WITNESS SAID THAT IMMEDIATELY AFTER THE TURN WAS COMPLETED, THE ENG SPUTTERED & THE ACFT SUDDENLY ENTERED A STEEP DSCNT & CRASHED. IMPACT OCCURRED NEAR THE APCH END OF RWY 28L. BFR GND IMPACT, THE ACFT HIT PWR LINES, CAUSING A PWR OUTAGE IN THE AREA. THE FUEL SYS WAS CONFIG- URED FOR BOTH TANKS TO FEED. ONE FUEL TANK WAS FND EMPTY, THE OTR HAD APRX 5 GAL REMAINING; HOWEVER, THE AMOUNT OF FUEL THAT MAY HAVE LEAKED AFTER THE CRASH WAS NOT VERIFIED. AN ENG TEAR DOWN SHOWED NO INDCN OF A PREIMPACT MECH FAILURE, BUTTHE CARB HEAT WAS FND IN THE 'OFF' PSN. TEMP & DEW POINT WERE 47 & 45 DEG, RESPECTIVELY. ICING PROBABILITY CHARTS SHOWEDTHAT SERIOUS CARB ICING COULD HAVE OCCURRED WHILE AT A CRUISE OR GLIDE POWER SETTING.