WHILE THE ACFT WAS EN ROUTE, ATC ADVISED THE PLT OF VFR TRAFFIC AT HIS ONE O'CLOCK POSITION AT 12 MI ON A NW HEADING. THE PLT ACKNOWLEDGED, BUT REPORTED THAT HE HAD NO CONTACT & SAID THE WX WAS A LITTLE HAZY. A SHORT TIME LATER, ATC ADVISED THE TRAFFIC WAS CLEAR & THE PLT ACKNOWLEDGED. THIS WAS THE LAST TRANSMISSION FROM THE ACFT. AT ABOUT THE SAME TIME, A PA-28 PLT ON A 150 DEG HDG SAW AN ACFT (N999YP) AT HIS ONE O'CLOCK POSITION ON A EASTERLY HDG, ABOUT 1 MI AHEAD. HE REPORTED THAT THE ACFT WAS IN A SHALLOW NOSE-DOWN PITCH ATTITUDE & APPEARED TO BE TRAVELING VERY FAST. WHEN THE ACFT MOVED TO HIS 11 O'CLOCK POSITION, IT CAME APART IN THE AIR AT AN ALT OF ABOUT 5000 TO 5500 FT. THE WRECKAGE WAS FOUND FRAGMENTED & SCATTERED OVER A 3000 FT AREA ON AN EAST-WEST DIRECTION. THE STABILATORS HAD SEPARATED FROM THE EMPENNAGE & THE WINGS HAD SEPARATED FROM THE FUSELAGE. THE WING CENTER SPAR WAS FOUND BENT & HAD FAILED FROM OVERLOAD. WX STUDY SHOWED PROBABLE LGT TO MOD TURBC AT 7000 TO 8000 FT MSL. SEAT BELTS/SHLDR HARNESSES WERE UNFASTENED/UNDAMAGED.