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Allan Pinkerton founded the first detective agency in Chicago, Illinois.
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A french police officer, Alphonse Bertillon was the first to use Anthropometry ( the science of the measurements of the human body) for criminal detection uses.
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Juan Vucetich invented the earliest recorded use of fingerprinting for criminal investigation uses.
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August Vollmer was the head of the Berkeley Police department and was integral in implementing the use of the polygraph and fingerprinting in police training.
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Calvin Goddard discovered the differences between a police bullet and a mob bullet after the Saint Valentine's Massacre. This discovery led to advancements in weapon identifying technology.