A doctor and her four children who were killed in a suspected arson attack may not have been the intended targets, police have said.
Sabah Usmani, sons Sohaib, 11, and Rayan, six, and 12-year-old daughter Hira, died in their house in Barn Mead, Harlow, Essex, in the early hours of Monday morning.
Her husband, Abdul Shakoor, also a doctor, suffered minor injuries as he fought to rescue his family from the blaze.
He is not being treated as a suspect and is said to be traumatised by the loss of his family.
Dr Sabah Usmani with her five children
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.
It is understood Dr Shakoor has been transferred to the Broomfield to be closer to his daughter.
Essex Police has said a possible line of inquiry is that the family were not the intended target of the attackers. A "painstaking" forensic examination has begun.
Detectives 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.
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."
Detective Superintendent Rob Vinson, who is leading the investigation, said the local community and Islamic groups could hold the answer.
He added: "I fully appreciate the community will be devastated. Nobody expects such a catastrophic incident in their own neighbourhood."
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.
Parvez Hamid, 43, and Safia Anwar, 38, said they had known the couple since they moved to Harlow and some of their children attended the nearby Abbotsweld Primary School together.
Mrs Anwar said: "They were a wonderful family. Their children were best friends with our own.
"We would see them at mosque and at the school gates.
"I last saw Sabah on Friday. She seemed normal with no worries at all."
Mr Hamid said: "Their children were so well behaved and I was always envious that mine weren't the same.
"They were hardworking parents who will be missed."
Melanie Walker, chief executive of the Princess Alexandra Hospital, said staff were "deeply saddened" by the tragic circumstances.
She added: "Our thoughts are with our colleague and his family at this very difficult time.
"Our staff, many of whom helped deal with this situation, are receiving the support they need."
Neighbours raised the alarm after being woken in the early hours.
Sharon Pavey has lived in the house next door for 10 years.
She added: "I saw flames coming out of the back window and dialled 999 but they were already on their way.
"The flames and smoke were billowing out and we could tell it was very serious."