Authorities in France’s second largest prison claim to have uncovered a plot by two inmates, who are thought to be Islamic State sympathisers, to plan acts of terror. They were able to communicate with the terror group using smuggled mobile phones.

Charles-Marie M. from Cameroon, 28, and a 22-year-old Frenchman named Maxime O. were scheduled to be released from Fresnes prison in the coming weeks. Investigators say the pair were actively recruiting and plotting an Islamist terror attack with individuals outside the prison to be implemented soon after their release, L’Express reports.

Unbeknownst to the two inmates, they were under surveillance by French intelligence services which led to their placement in police custody on October 2nd by the Directorate General of Internal Security (DGSI).

“The plot was not precise, but in their discussions, they talked about prison guards or policemen. They wanted to set up a group with other ‘fighters’,” a source close to the investigation said.

Cameroon native Charles-Marie M., who was in prison for robbery, first caught the attention of French intelligence in the winter of 2016 after authorities caught him communicating with a French jihadist operating in Iraq and Syria. Prison authorities say the man showed little sign of radicalisation with one prison supervisor describing him as “quite cold, with an empty look”.

The communication was not the first time the 28-year-old had shown signs of radicalism. In 2015, he was forced to spend six months in solitary confinement after guards found a USB stick in his possession containing jihadist propaganda.

Guards also noted that both men dressed in a Western style and did not bring attention to themselves or their beliefs. “The prison is nothing more than a microcosm of what is happening outside. The most dangerous go unnoticed,” one official said.

French prisons have become a breeding ground for Islamic radicalisation according to some experts. Muslims make up a large section of the prison population in the country and according to French newspaper Le Figaro, last year Muslims make up around 60 per cent of parolees.

Paris Attacker Greeted with Cheers in French Prison https://t.co/etQ6wp9F27 pic.twitter.com/BThQtmJOzU — Breitbart London (@BreitbartLondon) April 29, 2016

Last year, when one of the participants in the 2015 Paris terror attack was arrested and placed in a French prison he received cheers from inmates.

Belgian politician and European Union (EU) Counter-Terrorism Coordinator Gilles de Kerchove said that jihadists returning from fighting in the Middle East should not be placed in regular prisons at all because it may only further radicalise them.

Austrian anti-mass migration Freedom Party leader Heinz-Christian Strache has called for separate prisons on islands for convicted jihadists to stop them radicalising other inmates.