Francisca Rojas: The First Killer Convicted by a Fingerprint

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On June 29, 1892, police in Necochea, in Argentina's Buenos Aires province, found six-year-old Ponciano and four-year-old Felisa dead in their home. Their mother, 27-year-old Francisca Rojas, lay nearby with a wound to her neck and a ready accusation: a neighbor, Pedro Ramon Velasquez, had broken in and attacked the family. In an age when a surviving victim's word was the strongest evidence a court could ask for, the case looked settled. Then the neighbor's story held up, hers did not, and the local investigation stalled.

Word of the crime took until July 8 to reach La Plata, which sent Inspector Alvarez to break the deadlock. This episode follows his work, from an alibi check and a town rumor about a boyfriend's ultimatum to a brownish smudge on a bedroom door. Alvarez had trained under Juan Vucetich, a pioneer of the untested idea that friction ridges identify a person. What he did with that smudge ended one woman's lie and pushed an entire country to abandon the measuring calipers of the Bertillon system.

• Witnesses placed Velasquez out socializing with friends during the killings, collapsing the story of a home invasion.
• Alvarez learned that Rojas's boyfriend was reportedly willing to marry her on one condition: he wanted no part in raising her children.
• More than a week after the crime, on a scene long since trampled, Alvarez spotted a bloody fingerprint on a bedroom door and sawed out that section of wood to carry back to La Plata.
• When her inked prints matched the mark, Rojas confessed to killing her children and cutting her own neck to stage the scene.
• After the conviction, Argentina became the first country to abolish anthropometry and rely solely on fingerprint classification, using the filing system Vucetich designed.