Three persons, including a woman, of the same family were charred to death in their sleep after fire broke out at their two-storey home in Asansol on Saturday night.
Another family member was admitted to a private hospital in Durgapur with burns.
Police identified the deceased persons as Bablu Singh, 51, his relatives Gayatri Chand, 57, and Birendranath Chand, 62. Doctors said Bablu's wife, Shilpi Chatterjee, had suffered almost 60 per cent burns and her condition was serious.
Bablu, a coal trader, used to live in his own two-storey house in Asansol's Baishali Park area.
Police and fire brigade officials suspect the fire was caused following a short circuit in an air-conditioner. However, the forensic report is awaited to confirm the cause.
The forensic experts visited the spot on Sunday morning to collect samples and examine the bodies before they are sent for post-mortem.
Residents in the area said the fire broke out around 11.30pm on the first floor of the house where Bablu's relatives used to sleep in a room. Soon, the fire spread to the ground floor where Bablu was asleep in a room with Shilpi.
Local people informed the police.
"We saw flames billowing out of the house and immediately informed the police. We tried to douse the blaze by pouring water in buckets but failed. The entrance door of the house was closed," said Anjali Sanyasi, a neighbour.
A fire tender was pressed into service. Firefighter doused the blaze within half an hour. The firefighters broke open the door but meanwhile three persons were found completely charred in two rooms while Shilpi was found in a gasping condition and was rushed to the hospital.
Neighbours said that Bablu used to stay with his wife and relatives. Their only son Tirthankar studies in a school in Durgapur.
Police said Bablu, who originally hails from Jharkhand, owns another house in Nirsa of Dhanbad where his elderly parents, his former wife and two sons stay. "We have communicated with Bablu's family members (in Jharkhand). Initially we suspected a short-circuit behind the fire incident but a forensic test has been done to ascertain the actual reason behind the blaze," said Dhruba Das, deputy commissioner of police (central).