On 19 July 1936, after the military coup in Barcelona had been quashed, the Poor Clares left the Convent of Santa Maria de Jerusalem. The Iberian Anarchist Federation (FAI) militias then occupied the convent with the approval of the Central Committee of Anti-Fascist Militias (CCMA) and turned it into a detention centre to imprison those arrested by the Control Patrols responsible for ensuring revolutionary order. This centre operated on the margins of the law, and was often used to detain prisoners before they were executed in Montcada cemetery.

The convent had a number of rooms in the basement that met the requirements to turn it into a detention centre. The place was isolated and had a large amount of space, a vegetable garden with farm animals, two towers and high walls that surrounded the entire site. After the Events of May 1937, management of this detention centre was handed over to the Military Investigation Centre (SIM). The work carried out by the SIM in the rearguard in Barcelona put an end to the fifth column – Francoist collaborators – and augmented as the Republic was losing the war and the phenomenon of ambushes, fugitives and deserters among the Republican ranks increased