Using the recent terrorist attacks in Paris as a focal point, the WP found systemic problems in the French penal system whereby common criminals, such as Amedy Coulibaly a petty thief incarcerated in the Fleury-Merogis prison for a robbery in 2005, became terrorists. There they meet convicted terrorists such as Djamel Beghal, an Islamist with ties to al-Qaeda convicted of plotting to bomb the US embassy in 2001. Such convicts exercise a powerful influence on the general population in the prison. Their influence helped forge his (Coulibaly) belief in the call to jihad.
Following his release, Coulibaly killed a policewomen and four Jewish hostages in a Kosher deli as a mujahid in support of ISIS.
threat of radicalization:
"France’s prisons have a reputation as factories for radical Islamists, taking in ordinary criminals and turning them out as far more dangerous people."
Unfortunately it's recommended solution is to hire more religious workers without a structured process of certifying the Islamic clergy, something the IG's report believed was crucial in thwarting the radicalization process.
Without those safeguards unvetted prison Imams would only exacerbate the problem.