Some say it was the death of an inmate, others claim it was the arrest of a Muslim woman wearing the full veil, while the police believe the violence in Toulouse was a result of their good work. But the real cause is likely an longstanding problem.
More cars were burned in Toulouse on Wednesday night as unrest in the deprived Mirail neighbourhood continued for a fourth night.
On Sunday night gangs threw stones at the local police station and set fire to about 10 cars, violence flared again on Monday night and each night since.
The mayor of Toulouse threatened to impose a curfew on young people but in the end was persuaded that the violence was calming down.
There are various theories around what sparked the rioting in the Mirail area of the southern city, a high-crime neighbourhood once home to several jihadists.