What do the perpetrators of the recent terrorist attacks in Paris and Copenhagen have in common? Beyond the similarity of their chosen targets, three of the four men were homegrown Islamist extremists radicalized in European prisons.

Paris shooters Cherif Kouachi and Amedy Coulibaly met at French prison Fleury-Merogis, Europe’s largest jail, holding more than 4,000 inmates.  Kouachi was arrested and imprisoned for attempting to join jihad against American troops in Iraq in 2005.  Coulibaly was serving his third sentence for armed robbery. In their youth, both men had formed associations in Islamic radical circles.

The two men were housed in the same wing of the prison and were influenced by Djamel Beghal, a radical jihadist with ties to Osama bin Laden.  Beghal had been sent to France decades before to set up a terrorist cell. He was serving a 10-year sentence for a plot to bomb the U.S. embassy in Paris and became famous within the prison’s walls. Although Beghal was kept in isolation, the men found ways to contact him. It was during this period that Coulibaly converted to Islam.

Cherif Kouachi was again jailed in 2008 for helping to send militants to Iraq. Coulibaly also served an additional sentence in 2013 for his involvement in the effort to free Islamist Smain Ait Ali Belkacem from prison. Coulibaly was released in 2014. Instead of reforming the pair, prison had served as a nurturing environment for their growing extremism. Their ideologies continued to fester and intensify, coming to a head in Paris in early January 2015.

Twenty-two year old Omar Abdel Hamid El-Hussein, the perpetrator of the Feb. 15 Copenhagen attacks, was born and raised by Palestinian parents in the Danish capital. In January 2014, he was arrested and imprisoned on burglary charges and the stabbing of a 19-year old man. While in prison, he was radicalized to the point of pledging allegiance to ISIS, which he did on his Facebook page just days before the attacks. Prison authorities warned that El-Hussein was at “risk of being radicalized in jail,” but Danish intelligence services did not see him as an imminent threat. Two weeks after he was released, he carried out the deadly attacks that killed two and wounded five..

These perpetrators illustrate the severity of Islamic radicalization in European prisons. Today, Muslims constitute more than half of the population in French prisons, though they comprise only 8 percent of the French population. This makes it easy for radical Islamist prisoners to prey on potential recruits, rallying for their larger cause. EU governments will not be able to put a significant dent in extremism until they find a way to effect change in the prison system that today transforms so many into violent jihadists.