AFTER DEPARTING SAN JOSE, CA, THE PLT CLIMBED TO AN ALT OF 4600 FT MSL & PROCEEDED TOWARD NOVATO, CA. FROM 1528 TO 1533 PST, RADAR DATA SHOWED THAT THE ACFT WAS LEVEL AT 4600 FT ON A HEADING OF 330 DEG AT 180 FTS GROUND SPEED. RADAR INFO SHOWED THAT THE ACFT THEN ENTERED A DESCENT. DURING A 24 SEC PERIOD, THE ACFT DESCENDED TO AN ALT OF 4100 FT MSL & ITS HEADING CHANGED 10 DEG TO THE RIGHT. THE LAST RADAR POSITION, 12 SEC LATER, REVEALED AN ADDITIONAL HEADING CHANGE OF 150 DEG TO THE RIGHT, ACCOMPANIED BY A DESCENT TO 2300 (9000 FRM DESCENT). WITNESSES SAW THE ACFT DESCENDING AT HIGH SPEED IN A RIGHT TURN WITH THE ENGS AT HIGH POWER. JUST PRIOR TO IMPACT, THE RIGHT BANK ANGLE & NOSE LOW ATTITUDE INCREASED. THE ACFT CRASHED IN A STEEP NOSE DOWN, RIGHT WING LOW, ATTITUDE. NO PREIMPACT,MECHANICAL MALFUNCTION OR FAILURE WAS FOUND. NO AUTOPSY WAS WAS MADE, ONLY A GROSS EXAM OF THE PLT'S FRAGMENTED BODY WAS POSSIBLE. BEFORE DEPARTING SAN JOSE, THE PLT HAD REMARKED THAT HE WAS BOTHERED BY A BAD COLD.