Six labourers died after getting into a sewage-filled well inside a factory in West Bengal's North 24 Parganas district, police said on Friday.According to police, poisonous gas inside the well might have caused their death."Six labourers got inside a well in a paper factory in Naihati's Hazinagar area for cleaning work on Thursday evening. All of them were taken out in an unconscious state and later declared dead in hospital," an officer from Naihati police station said.According to eye witnesses, two of the labourers first went into the long disused well filled with sewage and other waste material from the factory. When there was no response from them, the other four went in to rescue them but none returned.Police was informed and police and fire brigade personnel rushed to the spot and pulled out the six unconscious men from the well."All six of them were sent to a hospital in Kalyani where four of them were declared dead. The remaining two also died at night," the officer said.Police said lack of oxygen or presence of poisonous gas might have killed the labourers as they did not have any masks or protective gear."A case has been registered against the factory owners for making the labourers work without any precaution," police added.