Experts have gathered in Brussels to advise the EU on how to tackle radicalisation in prisons.
Some of those who've launched attacks in Europe were radicalised in jails, and has prompted the bloc to look seriously at how to tackle the grass-root causes of extremist violence.
Is de-radicalisation possible?
"We cannot necessarily change what people think but we can affect and influence how they act on those thoughts. It's that risk, because that's what causes harm and causes threat to other people," said Gerry McNally, President of the Confederation of European Probation.
Several prison officers in France have recently been attacked by inmates, some of them suspected as having been radicalised.
Former Guantanamo inmate speaks out
One former Guantanamo prisoner told Euronews that jail conditions can play a big role.
"Prison is not a neutral place, it's a difficult place for people who are there. So some of them become more religious for example because religion helps them hold on," explained Mourad Benchellali.
"And as they become more religious they have questions about religion, legitimate questions, for which they find the answer where they can. So if they come across a radical person that's when the problem starts."
The EU's been worried that prison could become a breeding ground for terrorism and is funding various projects to help minimise the risks.