Antiterrorism prosecutors in Paris are investigating whether Lunel has been infiltrated by a jihadist cell or underground group like the so-called Buttes-Chaumont network, which once sent fighters to battle United States troops in Iraq and which was the starting point for at least one of last week’s Paris gunmen, Chérif Kouachi.

Mayor Arnaud said he doubted the existence of a network in Lunel, believing that those who left were just a group of school friends who decided to take up jihad after being influenced by radical websites as well as peer and family pressures. The six killed in Syria — ranging in age from 19 to 28 — all seem to have known one another.

“We are all trying to understand but feel completely out of our depth,” he said, flicking through a file prepared by the municipal police with the names and brief biographies of each of the deceased. All were born in France, he said, and none had previously attracted the attention of the local police.

One of those who left, Raphael Amar, is the son of a Jewish engineer. He grew up in a comfortable home with a swimming pool, played the guitar and loved Led Zeppelin. Last July, after traveling to Paris for a job interview, Mr. Amar, a 22-year-old convert, vanished, calling home a few days later to announce that he had gone to Syria, not to fight but to do relief work. He was killed just three months later in clashes at Deir al-Zour, in eastern Syria.

The same battle also claimed the lives of two of his school friends from Lunel, a town that in the medieval era and later attracted a large number of Jewish immigrants, particularly from Spain, but is today about a third Muslim.

“From almost one day to the next Raphael rejected everything,” said a person close to the family, who asked not to be named. “It was as if he had joined a sect.”