AN FBO MECHANIC REPORTED THAT THE PLT HAD REQUESTED A JUMPSTART. THE WX WAS COLD (20 DEG) & THE INITIAL ATTEMPTED ENG START WAS UNSUCCESSFUL. AFTER BEING PREHEATED, BOTH ENGS WERE STARTED. NOTHING ABNORMAL WAS NOTED DURING TAXI, RUN-UP & THE TAKEOFF ROLL. HOWEVER, DURING THE INITIAL CLIMB, BLACK SMOKE WAS OBSERVED TRAILING FROM THE LEFT ENG. AT APRX 600 FTAGL, THE SMOKE STOPPED & THE ACFT ENTERED A SHALLOW LEFT TURN & STARTED TO LOOSE ALT. THE PLT DECLARED AN EMERGENCY & SAID HE 'LOST' HIS LEFT ENG. SUBLSEQUENTLY, THE ACFT WAS OBSERVED TO ROLL, ENTER AN UNCONTROLLED DESCENT & CRASH. WITNESSES HEARD THE SOUND OF ENG POWER, BUT COULD NOT TELL WHETHER IT WAS FROM 1 OR 2 ENGS. NO PREIMPACT, MECHANICAL FAILURE WAS FOUND. THERE WAS EVIDENCE THAT THE GEAR WAS EXTENDED, THE FLAPS WERE UP & BOTH PROPS WERE AT A LOW PITCH (UNFEATHERED) SETTING. THE LEFT PROP BLADES HAD LITTLE ROTATIONAL DAMAGE. A TEST OF THE PLT'S BLOOD SHOWED: COCAINE 0.25 MG/L, BENZOYLECGONINE 3.78 MG/L, MORPHINE 0.32 MG/L, CODEINE 0.07 MG/L & CO2 9.2 TO 18%. THE PLT WAS A HVY SMOKER.