Since the riots, “there are some new buildings and parks,” said Ms. Cissé, 29. “But after that, what? What?”

The unemployment rate is 40 percent in Clichy, one of the poorest towns in France, with half the population under 25; there is a housing shortage, and there was no unemployment office here until last year, she said. It is only about 10 miles from central Paris, but there is no major road and still no mass transit station, despite a decade of promises.

“In no other developed country one sees towns so close but at the same time so far away,” she said. “One can’t continue to be 15 kilometers from Paris as the bird flies and yet need at least an hour and a half to reach the capital.”

The separation and alienation of youth is inevitable, she suggested. Some French lawmakers said, “These youngsters no longer deserve to be French because they no longer adopt the values of the republic,” she said.

“But France must assume its cosmopolitanism,” she continued. “One can’t welcome all sorts of people in the country without accepting their differences.”

Gilles Kepel is the great political scientist of the banlieues, having written three books about them, most recently “Passion Française — The Voice of the Ghettos,” as well as a report for the government. For him, the target of Islamic radicals is not France, per se, but Europe and its growing Muslim minorities. France, with the largest number of Muslims in Europe and the largest number of those traveling to Syria and Iraq for jihad, is an obvious focus.

But the effort to radicalize “nonetheless builds on things that are French,” Mr. Kepel said. Not just poverty and exclusion, he said, but the offense taken “even by Muslims who drink wine” over the cartoons of Muhammed, and then the French Republic’s full support for the cartoonists. As important, he said, is the unhealed wound of the French-Algerian war of independence and then of the Algerian war against Islamists.