Detectives investigating a house fire which killed six members of the same family are treating the deaths as murder, police have said.
A third son, Sohaib, 11, and daughter Maheen, three, died later in hospital.
Dr Sabah Usmani with her five children
Essex Police said: "Evidence from fire scientists has indicated that the likely cause of the fire was malicious ignition.
"Detectives know that there was a burglary at the house on the same evening/early morning of the fire and are re-appealing for information and witnesses."
A police spokesman said that from the start of the investigation, named Operation Shakespeare, detectives from the Kent and Essex Serious Crime Directorate had treated the incident as a suspicious fire.
He added: "A dedicated team of detectives and specialist police officers and staff have undertaken intensive inquiries, looking at all aspects of this case. They are working round the clock to establish how the fire started and who was responsible."
Senior investigating officer Detective Superintendent Rob Vinson said: "This is now a murder investigation. We believe that someone burgled the house and then caused a fire that took the lives of six innocent people.
"Someone knows something about this incident and for whatever reason has not come forward to tell police what they saw or know.
"Please search your conscience for the sake of those five young children and their mother. This is an unprecedented case in the history of Essex Police and we will do everything we can to solve this tragedy.
"Dr Shakoor has had to find the strength to cope with this terrible tragedy. No one can imagine the pain of losing your whole family in such a horrible way. I need to find answers. I need the public to tell me what happened on that night."
Dr Shakoor discovered the fire at his home and battled hard to save his family, suffering the effects of the dense smoke and also minor burns as he tried to get them out.
At the same time as fire crews were tackling the blaze, a Ford Focus car was alight in a parking area nearby.
Mr Vinson said: "I need the public's help and appeal for information about people who were in the area and about items taken in the burglary."
Dr Shakoor's laptop bag which contained a two-pin lead was originally found in a hedge along the footpath between St Michael's Close and Whitewaits, Harlow by two teenagers, then abandoned in Whitewaits by some garages.
"Do you know how the laptop bag came to be abandoned in the hedge?" Mr Vinson asked, adding that the family laptop, a Toshiba Satellite, was still missing.
He asked anyone who was in Barn Mead on the early morning of the fire to contact police.
"We need to find a group of four young men, 16-19, possibly wearing tracksuits and baseball hats, one significantly taller than the others.
Additionally we need to speak to two teenagers on bikes. Anyone who was in the area at the time of the fire or has any information about who was there must contact us."
He added: "The car that was discovered alight on the same morning belonged to a neighbour and was parked around 20 metres away from the Shakoor home. Do you have any information about this car fire?"
Anyone with information is urged to contact the Operation Shakespeare investigation team on 01277 266869 or 0800 056 0944 or on firstname.lastname@example.org or via Crimestoppers anonymously on 0800 555111.