DALLAS: A national transportation official probing the cause of a midair crash of two historic military planes during an air show that left six people dead said Sunday that one of the key questions for investigators is why the aircraft were seemingly sharing the same space just before impact.

A World War II-era bomber and a fighter plane collided and crashed to the ground in a ball of flames on Saturday, leaving crumpled wreckage in a grassy area inside the Dallas Executive Airport perimeter, about 10 miles (16 kilometers) from the city's downtown. Several videos posted on social media showed the fighter plane flying into the bomber.