https://en.wikipedia.org/wiki/1933_United_Airlines_Boeing_247_mid-air_explosion


Restored Boeing 247 in United Airlines livery, similar to the crashed aircraft. This one is on display at the Smithsonian Air & Space Museum

On October 10, 1933, a Boeing 247 airliner operated by United Airlines and registered as NC13304 crashed near Chesterton, Indiana. The transcontinental flight, carrying three crew and four passengers, had originated in Newark, New Jersey, with its final destination in Oakland, California. It had already landed in Cleveland and was headed to its next stop in Chicago when it exploded en route. All aboard died in the crash, which was proven to have been deliberately caused by an on-board explosive device.

Eyewitnesses on the ground reported hearing an explosion shortly after 9 p.m., and saw the aircraft in flames at an altitude of about 1,000 feet (300 m). A second explosion followed after the aircraft crashed. The crash scene 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 who combed through the debris 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 almost a mile away from the main wreckage. On November 16, 2017 the Federal Bureau of Investigation declassified 324 documents related to the investigation.

The incident
Melvin Purvis, head of the Chicago office of the United States Bureau of Investigation, described the damage, "Our investigation convinced me that the tragedy resulted from an explosion somewhere in the region of the baggage compartment in the rear of the aircraft. Everything in front of the compartment was blown forward, everything behind blown backward, and things at the side outward." He also noted: "The gasoline tanks, instead of being blown out, were crushed in, showing there was no explosion in them."

Investigation
An investigator from the Porter County coroner's office, Dr. Carl Davis, and experts from the Crime Detection Laboratory at Northwestern University examined evidence from the crash, and concluded that the crash had been caused by a bomb, with nitroglycerin as the probable explosive agent. One of the passengers was seen carrying a brown package onto the aircraft in Newark, but investigators who found the package amidst the wreckage ruled it out as being the cause of the explosion. A rifle was found in the wreckage but it was determined to have been carried aboard as baggage for a passenger who was en route to attend a shoot at Chicago's North Shore Gun Club. Despite the efforts of the investigators, no suspect was ever identified or charged in this incident, and it remains unsolved. This was the first proven act of air sabotage in the history of commercial aviation, although the earlier 1933 Imperial Airways Diksmuide crash may have been due to sabotage.

Pilot Captain Terrant, his co-pilot, flight attendant Alice Scribner, and all four passengers were killed. Scribner was the first United flight attendant to be killed in an aircraft crash.

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