Earlier, officials said that the abduction and murder were apparently not motivated by anti-Semitism, but added they have not yet discovered what led the group to commit the acts.
Authorities found 23-year-old Ilan Halimi naked, handcuffed and covered with burn marks last Monday near railroad tracks in the Essonne region south of Paris. He died on the way to a hospital, officials said.
Justice Minister Pascal Clement, speaking to reporters outside an annual meeting of France's main Jewish umbrella group, CRIF, confirmed investigators had alleged "aggravated circumstances of anti-Semitism."
One 19-year-old suspect was placed under investigation - a step short of formal charges - and locked up for suspected kidnapping and association with a group intending to kill, judicial officials said on condition of anonymity because the case is ongoing.
That suspect, however, was not accused with Halimi's killing.
Victim seduced by woman?
Five other suspects also placed under investigation were expected to appear later Monday before a judge who was to determine whether they too should be temporarily jailed. Investigating judge Corinne Goetzmann was deciding whether to place a seventh suspect under investigation, officials said.
After the killing, police said they believed that the ringleader behind Halimi's death was still at large.
The affair, which stunned France's Jewish community, began about a month ago, when an attractive young woman entered a cellular phone store in Paris, where Halimi worked, and began talking to him.
The two exchanged telephone numbers, and after speaking on the phone a number of times set a meeting in a suburb south of Paris. Halimi left for the meeting on January 20, and has been missing ever since.