On October 10, 1933, a United Airlines Boeing 247 propliner 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, but exploded en route. The crash is believed to be the first proven act of air sabotage in the history of commercial aviation.
An investigator from the Porter County coroner’s office and experts from the Crime Detection Laboratory at Northwestern University examined evidence from the crash and concluded that the explosion was the result of a bomb, with nitroglycerin as the probable explosive agent. (wiki)