European jihadists are using jail time to radicalize at-risk Muslim youth and forge connections with other terrorists, turning Europe’s prison systems into Jihad factories, The Wall Street Journal reports.

The only surviving member of the November 2015 Paris attacks was greeted with cheers after he was transferred to one of France’s most high security facilities. “Some welcomed him as the messiah,” Marcel Duredon, a security guard at the high security facility, told The Wall Street Journal.

France’s prison system in particular has become a breeding ground for would-be Jihadists. Approximately 50-60% of France’s prison population identifies as Muslim, despite Muslims only compromising 7.5% of France’s population. Prison officials are debating whether to integrate convicted jihadists with the prison general population, or to isolate them.

One of the terrorists who stormed a historic Normandy church Tuesday and slaughtered an 85-year-old priest was a known terror suspect and even wore an electronic tracking device. He reportedly told people he had met his “spiritual guide” in French prison, and that his prison mentor “gave him ideas.”

French authorities note that Abdeslam was convicted of petty theft, and served prison time with the mastermind of the Paris attacks, before they both traveled to Iraq and Syria to fight for ISIS. A French official responsible for auditing the prison system told the WSJ, “We’re sitting on a time bomb,” and that Islamic extremists inside prison act as an “aristocracy.”

A French audit also reportedly noted that many prisoners decorate their cells with ISIS flags. France has experimented with bundling known jihadists together, but have been unable to isolate them from passing messages to non-radicalized prison populations. France has 10,000 active suspects on its highest threat level terror watch list and less than 5,000 federal agents to surveil them, chairman of the French Center for the Analysis of Terrorism said in late June.

One former senior counter-terrorism official lamented to the WSJ, “We are putting together terrorists who didn’t know each other and belonged to different groups, and helping them create tight, impenetrable networks.”

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