In the context of the fight against Islamist radicalization in France, prison intelligence rapidly developed from 2015 through the gradual creation of a dedicated service and a specific corps of professionals. This professionalization of prison intelligence work has deeply transformed the prison administration. This article aims to describe and analyse these transformations on the basis of an ethnographic study conducted in radicalization assessment units, which are specific units set up to assess prisoners who have committed or are suspected of committing crimes linked to radical Islam. We shall describe how the guards, probation officers, psychologists and educators participating in assessing the prisoners adapt to the new, encroaching presence of the intelligence mission. We shall analyse the forms of collaboration and competition between this staff and the prison intelligence officers. Lastly, we will examine criticism of the intelligence activity in the radicalization assessment units voiced by various professionals. The interpenetration of the assessment work and the intelligence mission – which are formally distinct missions – produces a specific type of knowledge relating to radicalized prisoners: a reproduction of certain representations or ‘profiles’.