Two persons were killed and one more fell critically ill after they somehow inhaled lethal carbon monoxide at the Khardah-based factory of Electrosteel Castings Limited (ECL) on Wednesday afternoon.

Police and local sources said that a maintenance worker and a contractor accidentally came into contact with a volume of stored carbon monoxide inside a duct that was apparently unknown to them while undertaking a routine maintenance job during which production at the ECL factory had been suspended for a day.

ECL is one of the largest manufacturers of ductile iron pipes in the Indian sub-continent, and has one of its factories in Khardah of North 24-Parganas district in Bengal.

Workers claimed that early in the afternoon, a few workers had been cleaning a duct where contractor Swapnodip Mukherjee was supervising the job personally.

But at least two workers, Ranjit Singh and Rohit Mahato, and contractor Swapnodip Mukherjee suddenly fell ill.

Other workers on the job, on getting no response from them, alerted the factory officials who rescued the trio.

With the support of the other workers the trio were rushed to the ESI Hospital in Kamarhati, where Singh and Mukherjee were declared brought dead. Mahato is fighting for his life at a private hospital.

Mrinmoy Mondal, a worker at the factory, said: “It appears to be an accident. But it is not clear to us how it occurred. The company is very careful about the safety measures. Still, the accident occurred and claimed two lives.”

Repeated calls to the ECL factory office in Khardah and to its registered city office on Camac Street in Calcutta went unanswered.