Those planning to escape learnt from
Communist Party prisoners, who had been incarcerated longer, that there was a wall space which could be used in an escape attempt. Plans were made for 54 prisoners to escape on 2 August 1975, however, the escape was thwarted after an undercover agent learned of the plans. Following that, preparations began anew. At this point, the prison contained 53 political prisoners and 7 prison guards. The number of prisoners planned to escape was reduced due to the added security following the thwarted escape and the transfer of some of the original 54 planned escapees to other prisons. In total, 29 prisoners took part in the escape, 25 of these belonging to ETA, with the others members of leftist armed group
FRAP. In total, the prisoners had been sentenced to more than 1,500 years in prison. Priority was given to prisoners serving longer sentences. A camera had been smuggled into the prison for the previous attempt in order to photograph the participants, so that identity documents and false passports could be prepared. Parcels containing civilian clothing had also been smuggled into the prison. As with the August attempt, the escape was led by ETA, who carried out a detailed study of the prison to identify weak points which could be exploited in an escape. They concluded that the prison's drainage system would offer the best chance and began searching for the quickest way to reach it. Tests on the prison's walls revealed a hollow space behind the toilets and preparations for the escape attempt began in October 1975. Using a piece of metal taken from the prison's lumberyard, the prisoners removed six tiles to create a crawl space, through which they could reach the hollow space and begin tunnelling. To hide their work, they replaced the tiles and built a concealed door. Other prisoners assisted in the escape by making noise during the periods when the excavation of the tunnel was taking place. Having spent a month working to reach the hollow space, the prisoners' work became easier due to the hidden door. Up to five prisoners took turns digging a tunnel towards the exterior of the prison. After digging for two metres they found that they had entered the drainage tunnels below the prison and sawed through the bars to reach these. ==Escape==