PRIOR TO THE ACCIDENT, A WITNESS HEARD THE ACFT, THEN SAW IT FOR A SHORT TIME THRU AN OPENING IN THE CLOUDS. IN HIS OPINION, IT WAS HEADING DIRECTLY INTO A THUNDERSTORM, WHICH CONCERNED HIM. HE SAID THAT RAIN WAS FALLING, AND ABOUT 10 MIN LATER, THE THUNDERSTORM PASSED OVER HIS POSITION. SUBSEQUENTLY, THE ACFT CRASHED IN AN OPEN FIELD. THE LEADING EDGES OF BOTH WINGS WERE COMPRESSED BACK IN AN 'ACCORDIAN' MANNER AT AN ANGLE OF ABOUT 45 DEG, WHICH WAS INDICATIVE OF IMPACTING IN A 45 DEG, NOSE LOW ATTITUDE. THERE WAS A SMELL OF FUEL IN THE AREA & NO PREIMPACT MALFUNCTIONS OR FAILURES WERE FOUND. A METEOROLOGICAL STUDY SHOWED THERE WERE VIDEO INTEGRATOR & PROCESSOR (VIP) LEVEL 2 & LEVEL 4 RADAR ECHOS FROM THE THUNDERSTORMS & PROBABLE SEVERE TURBULENCE. A BOTTLE CONTAINING 600 MG OF MOTRIN (IBUPROFEN) WAS FOUND IN THE ACFT, BUT A MEDICAL EXAM REVEALED NO EVIDENCE OF A PHYSIOLOGICAL PROBLEM.