(Alternative source.)

During the transition to [neoliberalism], neither the State nor the judiciary investigated mass crimes connected to the repression and execution of left‐wing Republicans. […] Between 367,000 and 500,000 prisoners went through those camps, from the coup d’état in July 1936 until January 1947, when the last camp, the camp of Miranda de Ebro, ceased to operate.

[…]

In the early 1940s, around 370,000 political prisoners were in Franco’s jails. In 1942, this encouraged the régime to create a system of ‘remission of penalties through work’ — based on the Catholic concept of ‘expiration of duties through work’.

From a theoretical point of view, the crime was considered as sin and guilt was substituted by expiration as a form of prisoner’s conversion. Its main purpose was to expedite the movement of inmates by sending them to labour camps, and to generate a cheap labour force to be used by the “New State”. Also, private businesses benefited from it.

[…]

Inside the camp, a number of violent methods were used for the humiliation and disarticulation of the enemy. Survivors of the camp have reported extreme physical violence and psychological mistreatment. A further form of violence was the permanent lack of food and water for prisoners as a daily punishment.

Widespread famine was one of the main characteristics of the Francoist camps. Along with the problem of food, or lack of thereof, poor hygiene was another significant issue within the concentration camp, as well as the proliferation of lice, parasites and rats that contributed to the increase of numerous infectious diseases.

The harsh living conditions in concentration camps have been considered an additional form of punishment as prisoners were deprived of their liberty. These repressive centres utilised a technology of pain to achieve their aims of humiliation and punishment. In Castuera, archaeological research has shown various procedures established for the psychological degradation of prisoners, most clearly represented in the humiliating design of the latrines.

The latrines were designed to make inmates defecate publicly, in an open space, in groups, and with a disregard for gastrointestinal problems — this was another way to degrade and humiliate prisoners. The [parafascist] punitive system aimed to re‐educate prisoners in the Catholic faith and eradicate ‘Marxist ideals’.

It was for this purpose that a large cross was placed in a prominent position in the square of the camp dedicated to public events and re‐education activities, such as prayers, mass or the singing of the Francoist Hymn.

[…]

This grave contained the bodies of twenty‐two men that were immobilised, tied up in pairs at the wrist and elbow, two of them tied together at the neck. At least six short weapons were used for the executions. A wine bottle was also found thrown on top of the bodies in the mass grave. This extraordinary piece of evidence indicates that perpetrators, in this case, paramilitaries, may have been drunk during the executions — a common feature confirmed by multiple testimonies.

(Emphasis added.)

ETA: Discovery of a mass grave from the Spanish Civil War using Ground Penetrating Radar and forensic archaeology.