United Air Lines Flight 23 was a regularly scheduled flight operated by United Air Lines between Newark, New Jersey, and Oakland, California, with intermediate stops. On October 10, 1933, the Boeing 247 airliner serving the flight—registered as NC13304—exploded and crashed near Chesterton, Indiana, United States, en route from Cleveland to Chicago. It carried three crew and four passengers. All aboard died in the incident, which was caused by an on-board explosive device. Eyewitnesses on the ground reported hearing an explosion shortly after 9 pm and seeing the aircraft in flames at an altitude around 1,000 feet (300 m). A second explosion followed after the aircraft crashed. The crash site was adjacent to a gravel road about 5 miles (8 km) outside of Chesterton, centered in a wooded area on the Jackson Township farm of James Smiley.
Investigators combed through the debris and were confronted with unusual evidence; the toilet and baggage compartment had been smashed into fragments. Shards of metal riddled the inside of the toilet door, while the other side of the door was free of the metal fragments. The tail section had been severed just aft of the toilet and was found mostly intact, with two of the victims' bodies nearby, almost 1 mile (1.6 km) away from the main wreckage.
The Federal Bureau of Investigation declassified 324 documents related to the investigation on November 16, 2017. It is notable for being the first proven act of air sabotage in the history of commercial aviation.
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Text from the Wikipedia article “1933 United Airlines Boeing 247 mid-air explosion”, available under CC BY-SA 4.0.