The prime suspect in the German Christmas market attack was from a north African country that has become a breeding ground for extremism and is one of the top exporters of jihadists.

Tunisia was once thought to be an Arab Spring success story — it was called the “sole democratic success” of the uprisings that swept the Middle East starting in 2010.

But the country has been struggling with an extremism problem that was exacerbated with the arrival of the terrorist group ISIS, which has attracted thousands of Tunisian recruits. A 2015 study by The Soufan Group, a strategic security firm, found that Tunisia supplied more foreign fighters to ISIS than any other country, between 6,000 and 7,000 as of October of that year.

One Tunisian ISIS supporter drove a truck through a Christmas market in Berlin earlier this week, killing 12 people and injuring dozens more, according to authorities. Tunisia native Anis Amri, who was killed in a Friday shootout with police, was German authorities’ prime suspect in the attack. Officials had been investigating Amri, but after surveilling him for months, they couldn’t find any evidence of a specific plot.

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