"We think they directly took part in the deaths of the two victims," Jean-Yves Coquillat, the prosecutor of Grenoble said, adding that no one had admitted guilt.
Kevin Noubissi and Sofiane Tadburt died last Friday after being stabbed and beaten by a mob armed with hammers, baseball bats and knives in the southeastern city of Grenoble.
Twelve people had been placed in custody but four were released due to insufficient evidence, prosecutors said. Two fugitives are still on the run and one turned himself in late Wednesday.
Noubissi was stabbed seven to eight times, and Tadburt 30 times. The gang was reportedly seeking vengeance after a minor incident outside a high school involving Noubissi's younger brother.
Funerals for the two friends took place Wednesday at the local mosque in Grenoble and it drew hundreds of mourners.