Four of a family in Beckbagan were injured in an explosion possibly triggered when a family member switched on a mobile charger connected to a switchboard in a room filled with LPG gas because of an overnight cylinder leak, police said.

“Preliminary findings of forensic department officials suggest it was a vapour cloud explosion,” said a senior officer at Lalbazar. Three of the four victims have been hospitalised with burns.

Anand Das, 40, wife Kiran, 35, son Shivan, 7, and niece Bhumi Chowdhury, 16, were in the house on the ground floor of a five-storey building in Ahiripukur First Lane, close to Beckbagan. The incident happened after 5.30am on Thursday, one of their relatives said.

“I received a call early in the morning from my younger sister. She said there was an explosion in her home. I rushed over and found that their house had been damaged and my brother-in-law was seriously injured and covered in debris. My sister and nephew were also injured. Many people had gathered to see them but no one was coming forward to help them,” said Kiran’s elder brother Vijay Nayak.

A wall of the house and a portion of the main door exploded. Such was the intensity of the blast that a window grille was twisted out of shape under its impact.

Nayak said his sister told him that the family was sleeping when his brother-in-law Anand woke up and went to the washroom. On returning, he plugged his mobile phone to the charger and switched it on when the explosion happened.

Anand, who his landlady said works as a cook at a consulate, and Kiran have been admitted to a private hospital on Park Street. Their son has been admitted to SSKM Hospital and niece Bhumika was treated and discharged from the hospital.

Anand suffered the most severe burns, including on his hair and face. Kiran had burns on her hands while their son injured his hands and legs.

Investigators said they had found two LPG cylinders in the house, one of which was empty.

“According to a preliminary opinion of the forensic science laboratory (FSL), an electric spark could have triggered the explosion as the air was already heavy with leaked gas, possibly from the gas cylinder,” said an officer.

The switch board and the electrical connections of the house were also being examined by the FSL team, the officer added.