Two historic military planes collided and crashed to the ground in a ball of flames during a Dallas air show in the United States, leaving six people dead, officials said. National transportation officials were investigating the cause of Saturday’s collision, which came three years after the crash of a World War II-era bomber in Connecticut that killed seven, and amid ongoing concern about the safety of air shows involving older warplanes. Emergency crews raced to the crash scene at the Dallas Executive Airport, about 10 miles (16 kilometres) from the city’s downtown. News footage from the sce…