Complex sentence: It has not been possible to identify the Soviet pilot who shot him down, but it was not, as some sources suggest, high scoring Soviet ace Aleksandr Pokryshkin who was not in the area at the time.
Split into simpler sentences: It has not been possible to identify the Soviet pilot who shot him down. It was not, as some sources suggest, Aleksandr Pokryshkin who was not in the area at the time.