Early in July 1892 Inspector Alvarez of the police force in La Plata, Argentina, arrived in nearby Necochea to help investigate a particularly unpleasant murder. Two children had been found battered to death in bed in the shack where they lived with their mother, 26 year-old Francisca Rojas. Although suspicion had fallen on Francisca herself, the police had failed to extract a confession from her.

Alvarez searched the shack for clues and on the door found the print of a bloody thumb. Removing that piece of the door with a saw, he took it to the police station, then sent for Francisca and had her fingerprinted. When she learned that the print on the door matched that of her own right thumb, she broke down and confessed to the crime..

..Alvarez had studied the work of his colleague Juan Vucetich, head of the bureau of statistics for the La Plata police. Vucetich had discovered a method of analyzing and classifying fingerprints that made them easy both to file and, equally vital, to retrieve. Until then, the police authorities had largely ignored his work. But because of the Rojas case and subsequent successes, Argentina adopted fingerprinting as its sole method of identifying criminals – the first country in the world to do so.