PARIS -- For some, the step from feeling marginalized to taking up arms can be a short one.
At least five of the terrorists who launched Friday's attacks in Paris were French citizens, some of them born and raised there. The French president says his country is now at war, but the uncomfortable truth is that the enemy comes from within.
They're a tiny fraction of France's more than five million Muslims, but around a thousand French citizens are thought to have joined ISIS in Iraq and Syria -- and more than 200 are believed to have returned home.
Christian Prouteau used to lead the anti-terror squad in France's paramilitary police and has studied how young men are radicalized.
"Somebody convince them that this is right," Prouteau told CBS News, referring to "dying... in the name of God."
After the bloodshed, the French have shown unity in their grief. Muslim leaders have condemned the attacks.
Yet many Muslims in France say they feel excluded, ghettoized in poor suburbs and discriminated against because of their religion.
Kamal Masaudi told CBS News he immigrated to France four years ago from Algeria.
"It's not equal," Masaudi said. "For Muslims has bad picture here in France."
Only in a small minority of cases do those feelings turn to violent extremism. But Prouteau said fixing social problems is the only solution.
"They have no money, no work, and so on. And suddenly they think that perhaps they can find an aim in their, an action in their life," Prouteau said.
There was a sharp spike in anti-Muslim incidents in France following the Charlie Hebdo attacks in January. The fear now is that in the aftermath of these attacks, French Muslims will become even more isolated.