Tests carried out at the scene of a house fire which killed a woman and four of her children suggest that the blaze may have started accidentally.
Police investigating the blaze in Harlow, Essex, on Monday say they are not ruling any possibility out and are continuing to investigate the possibility that it was an arson attack.
However, early results suggest the blaze started in the lounge at the rear of the house and that no accelerant, such as petrol, was used.
The family used to live in Winchester, where the mother, Sabah Usmani, and father, Abdul Shakoor, worked as doctors.
Sabah Usmani, her sons Sohaib, 11, and Rayyan, six, and 12-year-old daughter Hira died in their end-of-terrace house in Barn Mead, Harlow, in the early hours of Monday.
A third son, Muneeb, nine, and second daughter Maheen, three, were rescued by fire crews and taken to hospital in a critical condition but Muneeb later died.
Mr Shakoor, suffered minor injuries as he fought to rescue his family from the blaze. Detective Superintendent Rob Vinson said officers were continuing to investigate ''many lines of inquiry''.
Forensic work at the property is nearing completion and experts have so far found no evidence that an accelerant was used to spark the blaze.
Essex Police said investigators could not yet draw final conclusions but the possibility of a tragic accident is now considered increasingly likely.
Mr Vinson said: ''We know already from the fire service and our own findings that temperatures would have been extremely high, with thick toxic smoke permeating the house.
''Initial indications show that the fire appears to have taken hold in the lounge area, which is at the rear of the property.
''The ferocity of the blaze has made the forensic work more challenging.'' Mr Vinson urged residents and bystanders to come forward urgently to help establish what happened.
''Although we have spoken with many people, there are still those who have not made contact with us,'' he said.
''Very probably they feel they have no information and do not want to waste police time, but, in reality, if you were at or near the scene of this incident we need you to come forward urgently.
''Naturally local residents are concerned about what happened and this investigation will not end until we have exhausted every line of inquiry.''
Work has begun to board up and secure the Shakoor family home.
Dr Shakoor and Maheen remain in the Broomfield Hospital in Chelmsford.
It is thought a funeral for his wife and children could be held in Harlow next week.
Post-mortem examination results announced at an inquest yesterday showed they all died of burns and smoke inhalation.