Police say a doctor who lost his wife and four children in a suspected arson attack on their home in Essex "fought hard" to save them.
Four of the victims - Sabah Usmani and her sons Sohaib, 13, and Rayyan, six, and 11-year-old daughter Hira - died in the blaze in Barn Mead, Harlow, in the early hours of Monday.
A third son, nine-year-old Muneeb, was rescued with his three-year-old sister Maheen. Both were taken to the town's Princess Alexandra Hospital in a critical condition but Muneeb died later.
Maheen remains critical and has been transferred to a specialist burns unit at the Broomfield hospital, Chelmsford.
The children's father, Dr Abdul Shakoor, works at the Princess Alexandra Hospital where he is being treated for minor burns and smoke inhalation. His condition is described as stable.
Dr Shakoor apparently escaped the fire by jumping from a first-floor window and is said by police to have "fought hard to save his family in appalling conditions". 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 other arson attacks on cars nearby, officers said.
One possible line of inquiry is that the family were not the intended target of the attackers.
Witness reports suggest between one and four people were in the area at the time. Such sightings will form a key part of the investigation, police said.
Assistant Chief Constable Gary Beautridge 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."
Mr Beautridge said Dr Shakoor was "in severe shock". He added: "Our work with (him) to establish some of the facts of this case will, inevitably, take time because of the emotional trauma he has suffered."
One neighbour told Sky News the doctor was in his dressing gown, screaming "get my family out" and had to be physically restrained from going inside the house.
Emergency services were called to Barn Mead at 1.48am. They found a burning car and an end-of-terrace house "engulfed" with flames coming out of the front and back doors.
Firefighters wearing breathing equipment battled "punishing" conditions inside before pulling out six people, Chief Fire Officer David Johnson, from Essex Fire and Rescue, told Sky News.
It is believed the fire started downstairs.
Mr Johnson said: "The first crews that arrived were actually confronted initially by a car that was alight so thought they were dealing with a car fire, but almost immediately became aware that there was a very developed house fire almost adjacent to where the car fire was.
"It was a very, very, hot, intense fire."
Because of its ferocity detectives are investigating whether it was started with some kind of fuel. The house has been sealed off for forensic examination.
Although the blazing Ford car found nearby was not connected with the family, it is also being treated as arson. Mr Johnson said it was highly likely the two fires were related.
Originally from Karachi, Pakistan, Dr Shakoor and his wife lived in Saudi Arabia, where their children were born, before moving to Manchester and then Harlow.
Mrs Usmani was also a doctor, although she stayed at home and cared for the children.
Parvez Hamid, 43, and Safia Anwar, 38, have known the couple since they moved to Harlow in 2011.
Their children attended the nearby Abbotsweld Primary School with some of the Shakoor children.
Mrs Anwar said: "They were a wonderful family. The children were best friends with our own.
"We would seem them at mosque and at the school gates. They were just such a lovely family.
"I last saw Sabah on Friday. She seemed normal with no worries at all.
"I heard what happened from a friend who lived nearby. It has come as such a shock to us all and it is still sinking in. We are absolutely devastated."
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. The Asian community is very tight knit in this area and we are all in shock."
Workers at the Princess Alexandra Hospital have told Sky News Dr Shakoor was a highly respected member of staff.
Chief Executive Melanie Walker said: "The hospital is deeply saddened by the tragic circumstances which have occurred."
Detective Superintendent Rob Vinson, who is leading the inquiry, said it was likely the fire was started deliberately and asked anyone with information to contact police.
He said the local community and Islamic groups could hold the answer, adding: "I fully appreciate the community will be devastated. Nobody expects such a catastrophic incident in their own neighbourhood."