NICE, France — There is the Nice of popular imagination, the old-world resort dotted with palm trees and cafes that look out on the Mediterranean Sea, suffused with an incandescent light prized for centuries by artists.

Then there is the other Nice, one that begins to show its face a few blocks inland from the seaside Promenade des Anglais, the majestic arc of a boulevard where 84 people were killed by a 31-year-old Tunisian immigrantat the wheel of a 19-ton truck. This Nice is home to many Muslim immigrants from North Africa, including a secular middle-class that has lived alongside non-Muslim French, and is also a place that local officials estimate has sent as many as 100 young people to fight in Syria with extremists.

“It is rare that these two worlds mix with each other except at the moment of festivities or of agreement, like the gatherings on Saturday,” said Feiza Ben Mohamed of the Muslims of the South, an organization that fights radicalization, referring to the public mourning for those killed in the truck attack.

“Yet the first victim was Muslim, and a good number of the victims were Muslims,” Ms. Mohamed added. “Just yesterday I was on the promenade reflecting on what had happened, and a journalist asked me if I was there to apologize in the name of Muslims. I said to him, ‘No, I came to weep for the dead like everyone else.’”

Nice presents a many-layered reality. It is at once one of France’s most popular tourist destinations, a stone’s throw from the glamorous watering holes of the rich and famous at Cap Ferrat and Monaco, and it has one of France’s largest populations of Tunisian origin. The fact that the driver of the truck that plowed through crowds celebrating Bastille Day was a Tunisian living in Nice has turned a spotlight on the contrasts.

The Muslim community itself is layered. There is a substantial sector of well-educated and integrated North Africans, but also a population that lives in bleak housing blocks in the city’s outlying districts, where in 2013 close to 40 percent of young people were unemployed, according to the French statistics office Insee.

It is in those neighborhoods, where resentment of the affluent waterfront world runs high, that recruiters for the extremist Muslims fighting in Syria operate. One of them, Omar Diaby, a resident of Senegalese origin now believed to be living in Syria, has been linked through an associate or associates to Mohamed Lahouaiej Bouhlel, the man who carried out the Bastille Day massacre, according to French news reports.

Mr. Diaby, several Muslims with knowledge of radical circles said, targeted 13- to 15-year-olds as recruits to join the fight in Syria and was notorious for using violent radicalization videos that made killing seem like a game. The French newspaper Le Monde and other French news outlets said the authorities had found cellphone records indicating that Mr. Diaby and Mr. Lahouaiej Bouhlel knew, or at least called, some of the same people.

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