A doctor who lost his wife and four children in a suspected arson attack on their home "fought hard" to save them, police said.
Dr Abdul Shakoor suffered minor injuries in the blaze, but his wife Sabah Usmani, who is also a doctor, sons Sohaib, 11, and Rayan, six, as well as 12-year-old daughter Hira, died in their end-of-terrace house in Barn Mead, Harlow, Essex.
A third son, Muneeb, nine, and daughter Maheen, three, were rescued by fire crews and taken to hospital in a critical condition, although Muneeb later died at the Princess Alexandra Hospital.
Dr Shakoor was said to be traumatised at the loss of his children. He is not being treated as a suspect.
Police are working to establish a possible motive for the attack. There had been no previous problems with racism in the area although there have been earlier arson attacks on cars nearby, police said.
A possible line of inquiry is that the family were not the intended target of the attackers.
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."
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.