France knew it had a problem with Islamic radicalisation in its prisons long before it became clear that two of the three gunmen who carried out the Charlie Hebdo attack in January last year had first met both each other and a dangerous al-Qaida-linked militant in the country's largest jail.

But it was not until after the shootings, which claimed 17 lives at the satirical magazine's offices and a kosher supermarket in Paris, that measures were announced to counter the mounting risk of the country's prisons becoming recruitment centres for many more homegrown extremists.

Amédy Coulibaly, who killed five people, and Chérif Kouachi, who with his brother Saïd shot dead 12 more, befriended each other and convicted extremist Djamel Beghal in Fleury-Mérogis jail south of Paris.

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