Firemen doused flames that engulfed a garage in Villiers before it could spread to a neighbouring garage and a nearby petrol station.
The two teenagers were riding a stolen motorcycle when they collided with a police vehicle in Villiers-le-Bel on Sunday, triggering the unrest.
The police were accused of fleeing the scene by about 100 youths who converged on the crash site at a housing estate.
Police said the motorcycle had hit a police car on a routine patrol.
Neither teenager was wearing a helmet, witnesses said.
An internal police investigation has been ordered for “involuntary manslaughter and failure to assist persons in danger”, a state prosecutor said.
The police investigatory body initially said the police car was unable to avoid the collision, due to the motorcycle travelling at high speed.
But it also said the police were at “serious fault” in terms of assisting the motorcycle’s riders, a police source said.
Omar Sehhouli, the brother of one of the victims, said the police had rammed the motorcycle and did not assist the youths.
He said it was “failure to assist a person in danger … it is 100 per cent a [police] blunder”.