Juan Díaz de Garayo y Ruiz de Argandoña was born on October 17, 1821, in Eguilaz, a village near the town of Salvatierra in Álava, Spain. He was the ninth child of farmers Nicolás Díaz de Garayo and Norberta Ruiz de Argandoña. Juan was illiterate and never received any formal education. He grew up in a hard-working family and lived a simple and austere life.

At the age of 14, during the First Carlist War, Juan's parents sent him to work in nearby towns such as Salvatierra, Alaiza, Ocáriz, Izarza, Añua, and Alegría-Dulantzi. He worked as a farmhand, shepherd, and coal miner. In 1850, he began working for a widow named Antonia Berrosteguieta, who lived in Vitoria. She needed a servant who had experience in farming and field management. Soon after, they got married. Antonia was known by the nickname "La Zurrumbona," which Juan also came to inherit.

Juan and Antonia had five children together, but only three of them—Cándido, Josefa, and Tomás—survived. Their marriage lasted until 1863 when Antonia passed away. Shortly afterward, Juan married his second wife, Juana Salazar. This marriage had many conflicts, especially between Juana and her stepchildren. As time went on, Juan's eldest son left home. The other two children spent more time away from the family.

Juana Salazar died of smallpox in 1870. Juan married for a third time soon after her death. This marriage was also tumultuous, as his new wife was an alcoholic. After five years, in 1876, Juan found her bedridden when he returned home from work. He called for a doctor, but there was nothing that could be done for her. One month later, he married his fourth and final wife, Juana Ibisate, who was an older widow.

Though his relationships with his later wives were not ideal, there is no evidence that he played a role in their deaths. When he was questioned about his third wife's suspicious passing, Juan stated, “No; I did not kill her, because if I had I would have declared it, as I have done with the others.”
 

Juan Díaz de Garayo y Ruiz de Argandoña, known as "The Sacamantecas," committed a series of murders in the Basque region of Spain between 1870 and 1879.

His first murder took place on April 2, 1870. He approached a well-known woman from Vitoria who had turned to prostitution. After they paid for their encounter, he offered her three reales, which she found insufficient. When she demanded five, Garayo attacked her, strangling her until she lost consciousness. He then drowned her in a stream and returned to the city. The body was found the next morning, but authorities archived the case due to a lack of evidence.

On March 12, 1871, Garayo murdered a widow whom he had met while she was begging. He invited her to join him, and after a brief sexual encounter, he strangled her when she protested about the payment he offered. Once again, the body was discovered the following day, but investigators could not find leads.

The third murder occurred on August 21, 1872, targeting a 13-year-old servant girl. After attacking and raping her, Garayo strangled her and hid her body in a ditch. The discovery of this crime caused an uproar in Vitoria, yet investigators struggled to find any clues that would lead to his capture.

His fourth victim was a 23-year-old woman he approached on August 29, 1872. After they had sex, he attempted to strangle her. When she stirred, he stabbed her with a hairpin. He left her body near a river and returned home. The panic in Vitoria increased as local women became scared to go out alone.

In 1873, Garayo tried to kill another prostitute, but she was able to scream for help, causing him to flee. In June 1874, he attempted to strangle a sickly beggar woman. She was able to defend herself and was not killed, but she identified him.

After a four-year break from killing, Garayo struck again on November 1, 1878. He entered a mill and tried to strangle the miller. She fought back, and he was forced to flee, leading to a brief imprisonment.

On August 25, 1879, he murdered María Dolores Cortázar, a 25-year-old woman he lured off the road. He stabbed her multiple times and hid her body. The following day, he killed another woman, Manuela Audícana, after a failed sexual encounter. He disemboweled her in a gruesome manner before hiding her body and returning to Vitoria.

The investigation into these last two murders was thorough. Witnesses described Garayo, which led authorities to connect him to earlier unsolved crimes. He was arrested on September 21, 1879, after being recognized in the street. Initially, Garayo denied the charges, but after days of interrogation, he confessed to all murders and attacks.

His trial concluded with a conviction. Garayo was sentenced to death and displayed no emotion upon receiving the sentence. His defense argued that he was insane, but multiple evaluations deemed him sane enough to understand his actions.

Juán Díaz de Garayo was executed by garrote on May 11, 1881. His body was displayed publicly for a time before burial in an unmarked grave, although his head was believed to have been preserved for medical studies. He became infamously known as "The Sacamantecas," a name derived from a folklore figure that was said to abduct and kill people.