BAGNEUX, France, March 3  Two strips of red-and-white police tape bar the entrance to the low-ceilinged pump room where a young Jewish man, Ilan Halimi, spent the last weeks of his life, tormented and tortured by his captors and eventually splashed with acid in an attempt to erase any traces of their DNA.  Ilan Halimi was remembered by 350 people at a gathering in Toulouse, France, center, on Feb. 24. He died of injuries he suffered in Bagneux.  Jack Guez/Agence France-Presse  Getty Images  The floor of the concrete room, in the cellar of 4, rue Serge-Prokofiev, is bare except for a few packets of rat poison, a slowly drying wet mark and a dozen small circles drawn and numbered in white chalk, presumably marking the spots where the police retrieved evidence of Mr. Halimi's ordeal.  Mr. Halimi, 23, died Feb. 13, shortly after he was found near a train station 15 miles away by passers-by, after crawling out of the wooded area where he was dumped. He was naked and bleeding from at least four stab wounds to his throat, his hands bound and adhesive tape covering his mouth and eyes. According to the initial autopsy report, burns, apparently from the acid, covered 60 percent of his body.  "I knew they had someone down there," said a young French-Arab man, loitering in the doorway of a building adjacent to the one where Mr. Halimi was held. He claimed to live upstairs from the makeshift dungeon but would not give his name or say whether he knew then that the man was a Jew. "I didn't know they were torturing him," he said. "Otherwise, I would have called the police."  But it is clear that plenty of people did know, both that Mr. Halimi was being tortured and that he was Jewish. The police, according to lawyers with access to the investigation files, think at least 20 people participated in his abduction and the subsequent, amateurish negotiations for ransom. His captors told his family that if they did not have the money, they should "go and get it from your synagogue," and later contacted a rabbi, telling him, "We have a Jew."  The horrifying death has stunned France, which has Europe's largest Muslim and largest Jewish populations. Last weekend, tens of thousands of people marched against racism and anti-Semitism in Paris, joined by the interior minister, Nicholas Sarkozy, and smaller marches took place in several other French cities, including Marseille.  In the wake of the riots that broke out in the immigrant-heavy Paris suburbs last fall, the case seems to embody the social problems of immigration, race and class that France has been facing with so much uncertainty. The emerging details raise deep fears of virulent anti-Semitism within the hardening underclass, and point to the decaying social fabric in which that underclass lives.  Those that the police say kidnapped and killed Mr. Halimi called themselves the Barbarians, and included people of different backgrounds: the children of blacks from sub-Saharan Africa and the Caribbean, of Arabs from North Africa, of at least one Persian from Iran, and of whites from Portugal and France.  The gang's leader was a tall, charismatic young man named Youssouf Fofana, 25, one of five children born in Paris to at least nominally Muslim immigrants from Ivory Coast. When he was a teenager, the family moved to the bleak neighborhood of 12-story concrete apartment blocks where Mr. Halimi was held.  Trouble started early. He studied plumbing at a local vocational school but by the age of 16 had already begun a series of run-ins with the police, eventually racking up 13 arrests for everything from theft to fencing stolen goods. In 1999, at the age of 19, he stole a car, beating the Portuguese owner who tried to intervene. He was arrested and sentenced to his only jail term, serving two years in prisons in Nanterre and Fleury-Mérogis, neither far from Paris.  He returned to his mother's apartment and used his prison credentials to assume the role of senior tough among younger, idle men and women, people in the neighborhood say. Lawyers familiar with the case suggest that this is when the seeds of the Barbarians were sown.  By 2004, the police say, he tried extortion, aiming at prominent French Jews. When that failed, the gang apparently turned to kidnapping, using young women as bait.  The Barbarians are thought to have been behind six attempted abductions, four of Jewish men, before succeeding with Mr. Halimi.
The gruesome kidnapping and murder of a Jewish man bares anew the racial and religious hatreds beneath the surface in France.