The suspected attacker entered the factory and set off several small explosive devices, the source said. A decapitated body was found nearby the factory, another source said.
"According to the initial findings of the enquiry, one or several individuals on board a vehicle, drove into the factory. An explosion then took place," said one of the sources.
"The decapitated body of a person was found nearby the factory but we do not yet know whether the body was transported to the place or not," added this source, adding that a "flag with Arabic writing on it was found on the scene."
Interior Minister Bernard Cazeneuve said he would go "immediately" to the scene, his office said.