THE PLT & PAX WERE ON A FLT FROM AMARILLO, TX TO LITTLE ROCK, AR. AN ENROUTE STOP WAS MADE AT HASKELL, OK AT APPR 1900 TO REFUEL. THE OWNER OF THE AFPT AT HASKELL STATED THAT THE PLANE ARRIVED IN A FAIRLY HEAVY RAINSTORM. AT THAT TIME, A WINTER TYPE COLD FRONT WAS MOVING EAST THRU TEXAS, OKLAHOMA & ARKANSAS. NUMEROUS THUNDERSTORMS & HEAVY RAINSTORMS WERE PRESENT & A TORNADO WATCH WAS IN EFFECT FROM 2130 TO 0100 FOR EASTERN OKLAHOMA AND WEST ARKANSAS. THE ARPT OWNER SUGGESTED WAITING OVER NIGHT & OFFERED ACCOMODATIONS, BUT SAID THE PLT & PAX WERE IN A HURRY. REPORTEDLY, RAIN WAS FALLING QUITE HEAVILY WHEN THEY WENT TO THE ACFT. THEY WAITED UNTIL THE STORM SUBSIDED THEN TKOF; HOWEVER, THEIR PLANNED ROUTE WOULD HAVE TAKEN THEM THRU THE SAME STORM. AT APPRX 2010 A WITNESS NEAR THE CRASH SITE HEARD VARIATIONS IN ENGINE PWR. HE WENT OUTSIDE & OBSERVED THE PLANE APPRX 10 TO 15 SEC BFR IT WENT OUT OF SIGHT THEN HEARD ON EXPLOSION & SAW A FIREBALL WHEN THE PLANE HIT A MTN SIDE AT NGT. NO PREIMPACT ENGINE FAILURE WAS FOUND.