Police are investigating whether a doctor and her four children who were killed in a suspected arson attack on their home were not the intended targets.
Sabah Usmani, sons Sohaib, 11, and Rayan, six, and 12-year-old daughter Hira, died in their end-of-terrace house in Barn Mead, Harlow, Essex, in the early hours of Monday.
Her husband, Abdul Shakoor, also a doctor, suffered minor injuries as he fought to rescue his family from the blaze.
A third son, Muneeb, nine, and daughter Maheen, three, were rescued by fire crews and taken to hospital in a critical condition but Muneeb later died at the Princess Alexandra Hospital.
Essex Police has said a possible line of inquiry is that the family were not the intended target of the attackers. Dr Shakoor, who was said to be traumatised at the loss of his children, is not being treated as a suspect.
Detectives are working to establish a possible motive for the attack. There had been no previous problems with racism in the area although there had been earlier arson attacks on cars nearby, police said.
Witness reports suggest between one and four people were in the area at the time. Such sightings will form key lines of inquiry, police said. A car was also found alight a short distance from the house. The vehicle was not connected to the family but the fire is also being treated as arson.
Assistant Chief Constable Gary Beautridge, head of the Essex and Kent serious crime directorate, said: "In the history of Essex Police seldom has there been an incident of this gravity in which five people, four of them children, have lost their lives.
"I would like to stress that the father of these children was in the property at the time and fought hard to save his family in appalling conditions. He is being supported by specially trained family liaison officers and, as you would expect, is in severe shock."
Dr Shakoor worked at the Princess Alexandra Hospital. Dr Usmani did not work as she cared for the children. Originally from Karachi, Pakistan, they had lived in Saudi Arabia, where the children were born, for more than a decade before moving to the UK.