Hridesh Shaw amid the debris of his home. A Telegraph picture

Cooking gas was pumped into the Shaw family’s room on GT Road before it was set ablaze on February 7, forensic experts have stated in their final report on the fire that killed three persons.

“The small room had become a gas chamber. It went up in flames as soon as there was a spark,” D. Sengupta, the director of the state forensic laboratory, told Metro on Sunday.

Police found a five-kg gas cylinder and a tube near the room’s drainpipe.

According to Sengupta, someone might have struck a matchstick to start the fire. “The findings will be sent to Howrah police immediately to help them file the chargesheet,” he added.

The owner of the room, Kashinath Jaiswal, and his associate Amarnath Prasad have been accused of plotting to vacate the premises for constructing a multi-storeyed building.

The fire at dawn claimed the lives of Kewala Devi 40, her husband Ram Bahadur Shaw, 50, and their daughter Deepali, 13. The couple’s elder son Hridesh, 18, was spending the night at his private tutor’s house. The younger son of the couple, Dilesh, 15, was in the room but suffered only minor burns.

A senior police officer said Jaiswal had been eyeing the property for a long time and wanted to build a multistoreyed building on the plot.

“He had asked the tenants to vacate their rooms but the Shaws refused to budge. Jaiswal then came up with the plan to wipe out the family,” he added.

Howrah police have not yet zeroed in on the people employed to carry out the job.