At 0921 Central time, the airplane experienced an in-flight breakup while maneuvering in a Level 5 and 6 thunderstorm. Instrument conditions prevailed at altitude; an IFR flight plan had been filed. The airplane was destroyed, and the Private pilot and single passenger received fatal injuries. A review of radar and performance data revealed that the airplane was at 16,000 feet msl when it entered an area covered by a Severe Thunderstorm Watch and which was the subject of a Convective SIGMET for a line of thunderstorms 40 nm wide. Tops of the thunderstorms were reported at 44,000 feet, with 2-inch hail, and possible wind gusts up to 60 knots.