In 1933, a Boeing 247 operated by United Air Lines exploded mid-flight and crashed near Chesterton, Indiana. The crash killed all seven people aboard, including four passengers and three crew members. Investigators concluded that a bomb caused the explosion, making this the first proven act of air sabotage in the history of commercial aviation. No suspect has ever been identified or charged in the incident. How did investigators determine that a bomb caused the explosion? Discuss