Francesca Rojas’ two young children are killed in their home in the small town of Necochea, Argentina. According to Rojas, a man named Velasquez had threatened her when she rejected his sexual advances earlier in the day. Upon returning home later, Rojas claimed to have seen Velasquez escaping out her open door. Once inside, she found both her six-year-old boy and four-year-old girl stabbed to death.

Police arrested and questioned Velasquez, but he denied any involvement, even after some rather painful interrogation techniques were used to obtain a confession. Law enforcement officials even tried tying him to the corpses of the children overnight. When that didn’t produce any results, Velasquez was tortured for another week. Still, he maintained his innocence throughout the ordeal.

Juan Vucetich, in charge of criminal identification at the regional headquarters, had been intrigued by the new theories of fingerprint identification and sent an investigator to see if the methods could help crack the case. Until then, the only other method of identification was the Bertillonage, named after its inventor, Alphonse Bertillon, who worked for the Paris police. This method involved the recording of body measurements in more than 11 different places. In an age when photography was very expensive, Bertillonage gave police their best chance of definitively identifying a person.

When the investigator examined Rojas’s house, he found a bloody thumb print on the bedroom door. Rojas was then asked to provide an ink-print of her thumb at the police station. Even with only a rudimentary understanding of forensic identification, investigators were able to determine that the print on the door belonged to Rojas. Using this new piece of evidence against her, detectives were able to exact her confession.

Apparently, Rojas had killed her own children in an attempt to improve her chance of marrying her boyfriend, who was known to dislike children, and then pegged the crime on Velasquez. She was sentenced to life imprisonment.