Detectives investigating the deaths of a doctor and five of her children in a Harlow house fire are looking for teenagers who were seen in the area at the time.
They also revealed that a laptop computer taken from the Barn Mead home in a burglary may be the key to finding those responsible for the arson.
Dr Sabah Usmani and son Sohaib, 11, and daughters Hira, 13 and Rayyan, 6, died in the fire on Monday, October 15.
Dr Usmani's husband Dr Abdul Shakoor was the only survivor.
Speaking at a press conference in Chelmsford today, detectives revealed the burglary for the first time and laid out a number of other leads which are being investigated.
The bag which had contained the laptop was found near lock up garages in Whitewaits, Harlow, on October 26.
A teenage boy aged around 15 to 16 wearing a tracksuit and a slim girl wearing a Burn Mill school uniform were seen dumping the bag.
They are also asking a group of youths who were outside the house in the early hours of the morning on which the fire broke out to come forward.
They are described as four boys, aged 16 to 19, wearing tracksuits and baseball caps. Two were on bikes and one had a hood up.
One is described as 15 to 18 years old, with ginger or light brown hair, wearing black jogging bottoms with stripes down the legs, a hip-length anorak style jacket with a hood, and was riding a light coloured bicycle.
Another, riding a black mountain bike, was 16 to 18 years old and wearing a light grey top.
Police are also looking for the laptop, and are asking anyone who has been offered, or seen, a Toshiba Satellite laptop, to get in touch.
Dr Shakoor's laptop had been purchased in Saudi Arabia and has a two-pin style power socket.
Speaking at the conference, Assistant Chief Constable Gary Beautridge, head of the Kent and Essex Serious Crime Directorate said: "This tragic incident is unprecedented in terms of its scale of loss and impact on the local community and our thoughts remain with Dr Shakoor at this difficult, and in his own words 'unimaginable', time in his life.
"When we launched our investigation last month, we predicted that this would be a protracted and demanding inquiry but pledged that we would explore every avenue and search for the truth in spite of the challenges we would be presented with.
"This remains one of the most complex cases the investigative team have ever dealt with but we remain steadfast in our approach and confident the answers to this inquiry lie within the local community.
"The cause of the fire Dr Shakoor's home in Barnmead,
Harlow remains under investigation. We are working closely with Essex Fire and Rescue service and independent fire scientists but to date we have been unable to identify the cause of the fire and we wanted to explain the difficulties we have faced in this area of the investigation.
A photograph of the lounge, which was earlier named as the likely source of the fire, was also released.
ACC Beautridge said: "The fire burned through the downstairs of the house at a very high temperature, destroying most of the contents.
"It took paint off the radiator and the fire and water used to extinguish it has penetrated the plaster and taken it back to the brickwork.
"Scientists have been sifting through the remains and have confirmed that to date there is no evidence of a flamable liquid being used. It is however, important to emphasise that detailed forensic testing remains ongoing. Behind the scenes detectives and specially trained staff have been working around the clock piecing together information and intelligence which has led us to this point"
A dedicated phoneline has been set up for members of the public who have any information on the teenagers or the laptop computer to contact the Operation Shakespeare team, on 0800 0560 944
