Hours after a fire in a residential building in Delhi's Pitampura left six people dead, police have registered an FIR against unknown people and questioned the building owner and his wife, officials said on Friday.

Police suspect the main cause of the fire could be a short circuit in a room heater on first floor of the building.

A fire broke out in a multi-storey building in the northwest Delhi locality at 8 pm on Thursday evening. Six people, from three families, including four women, died in the incident.

Deputy Commissioner of Police (northwest) Jitendra Meena said the forensic science laboratory (FSL) team has collected samples from the spot to ascertain the cause of the fire.

"At this stage we cannot rule out the possibility of the short circuit in the appliance," said Meena.

Another police officer said prima facie it appears a room heater that was left on for several hours could have led a short circuit.

The FSL team has collected samples, including the burnt part of the room heater, officer said, adding that the exact reason of the blaze is yet to be ascertained.

Police said that every house had an automatic door lock, which could be the reason the occupants could not be evacuated on time.

The officer said owner of the building, Subhash Chand Gupta, and his wife Mamta stayed in the upper ground floor. His son and daughter-in-law stayed in the first floor of the building.

At the time of incident, Subhash Chand's son and daughter-in-law were out of town, due to which the first floor was locked.

The ground floor had parking spots for the residents, while the second and third floor had been rented out.

The officer said that police have recorded the statements of Subhash Chand, his wife and other neighbour.

According to the information received from hospitals, those who died were identified as Rakesh Gupta (62), Renu Gupta (62), Shweta (30), Shanu Verma (27), Santosh (25) and Kirti (25).

Another resident, Sahil (34), had to be sent to the hospital but he was discharged after being given first aid.

Police said except Santosh -- who was a cook in one of the houses -- all other deceased were tenants and were second and third floor residents.

A case under sections 304A (causing death by negligence), 285 (negligent conduct) and 336 (endangering lives) of the Indian Penal Code was registered at the Maurya Enclave Police station, police said.

The six people, including four women, who died in the incident belonged to three different families, police said.